High thermal conductivity ceramic body

ABSTRACT

A process for producing an aluminum nitride ceramic body having a composition defined and encompassed by polygon A4F5F6F4 but excluding line F6F4 of FIG. 4, a porosity of less than about 10% by volume, and a thermal conductivity greater than 0.90 W/cm.K at 25° C. which comprises forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point F5 up to point F4 of FIG. 4, said compact having an equivalent % composition of Y, Al, O and N outside the composition defined and encompassed by polygon A4F5F6F4 of FIG. 4, heating said compact to a temperature at which its pores remain open reacting said free carbon with oxygen contained in said aluminum nitride producing an deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O and N is defined and encompassed by polygon A4F5F6F4 but excluding line F6F4 of FIG. 4, and sintering said deoxidized compact at a temperature of at least about 1830° C. producing said ceramic body.

The present invention relates to the production of a liquid phasesintered polycrystalline aluminum nitride body having a thermalconductivity greater than 0.90 W/cm.K at 25° C. In one aspect of thepresent process, aluminum nitride is deoxidized by carbon to a certainextent, and then it is further deoxidized and/or sintered by utilizingyttrium oxide to produce the present ceramic.

A suitably pure aluminum nitride single crystal, containing 300 ppmdissolved oxygen, has been measured to have a room temperature thermalconductivity of 2.8 W/cm.K, which is almost as high as that of BeOsingle crystal, which is 3.7 W/cm.K, and much higher than that of α-Al₂O₃ single crystal, which is 0.44 W/cm.K. The thermal conductivity of analuminum nitride single crystal is a strong function of dissolved oxygenand decreases with an increase in dissolved oxygen content. For example,the thermal conductivity of aluminum nitride single crystal having 0.8wt % dissolved oxygen, is about 0.8 W/cm.K.

Aluminum nitride powder has an affinity for oxygen, especially when itssurface is not covered by an oxide. The introduction of oxygen into thealuminum nitride lattice in aluminum nitride powder results in theformation of Al vacancies via the equation: ##EQU1## Thus, the insertionof 3 oxygen atoms on 3 nitrogen sites will form one vacancy on analuminum site. The presence of oxygen atoms on nitrogen sites willprobably have a negligible influence on the thermal conductivity of AlN.However, due to the large difference in mass between an aluminum atomand a vacancy, the presence of vacancies on aluminum sites has a stronginfluence on the thermal conductivity of AlN and, for all practicalpurposes, is probably responsible for all of the decrease in the thermalconductivity of AlN.

There are usually three different sources of oxygen in nominally pureAlN powder. Source #1 is discrete particles of Al₂ O₃. Source #2 is anoxide coating, perhaps as Al₂ O₃, coating the AlN powder particles.Source #3 is oxygen in solution in the AlN lattice. The amount of oxygenpresent in the AlN lattice in AlN powder will depend on the method ofpreparing the AlN powder. Additional oxygen can be introduced into theAlN lattice by heating the AlN powder at elevated temperatures.Measurements indicate that at ˜1900° C. the AlN lattice can dissolve˜1.2 wt % oxygen. In the present invention, by oxygen content of AlNpowder, it is meant to include oxygen present as sources #1, #2 and #3.Also, in the present invention, the oxygen present with AlN powder assources #1, 2 and #3 can be removed by utilizing free carbon, and theextent of the removal of oxygen by carbon depends largely on thecomposition desired in the resulting sintered body.

According to the present invention, aluminum nitride powder can beprocessed in air and still produce a ceramic body having a thermalconductivity greater than 0.90 W/cm.K at 25° C.

In one embodiment of the present invention, the aluminum nitride in acompact comprised of particulate aluminum nitride of known oxygencontent, free carbon and yttrium oxide, is deoxidized by carbon toproduce a desired equivalent composition of Al, N, Y and O, and thedeoxidized compact is sintered by means of a liquid phase containingmostly Y and O and a smaller amount of Al and N.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the figures accompanying and forming apart of the specification in which:

FIG. 1 is a composition diagram (also shown as FIG. 1 in U.S. Pat. No.4,547,471 showing the subsolidus phase equilibria in the reciprocalternary system comprised of AlN, YN, Y₂ O₃ and Al₂ O₃. FIG. 1 is plottedin equivalent % and along each axis of ordinates the equivalent % ofoxygen is shown (the equivalent % of nitrogen is 100% minus theequivalent % of oxygen). Along the axis of abscissas, the equivalent %of yttrium is shown (the equivalent % of aluminum is 100% minus theequivalent % of yttrium). In FIG. 1, line ABCDEF but not lines CD and EFencompasses and defines the composition of the sintered body of U.S.Pat. No. 4,547,471. FIG. 1 also shows an example of an ordinates-joiningstraight line ZZ' joining the oxygen contents of an YN additive and analuminum nitride powder. From the given equivalent % of yttrium and Alat any point on an ordinates-joining line passing through the polygonABCDEF, the required amounts of yttrium additive and AlN for producingthe composition of that point on the ordinates-joining line can becalculated;

FIG. 2 is an enlarged view of the section of FIG. 1 showing thecomposition of the polycrystalline body of U.S. Pat. No. 4,547,471;

FIG. 3 is a composition diagram showing the subsolidus phase equilibriain the reciprocal ternary system comprised of AlN, YN, Y₂ O₃ and Al₂ O₃.FIG. 3 is plotted in equivalent % and along each axis of ordinates theequivalent % of oxygen is shown (the equivalent % of nitrogen is 100%minus the equivalent % of oxygen). Along the axis of abscissas, theequivalent % of yttrium is shown (the equivalent % of aluminum is 100%minus the equivalent % of yttrium). In FIG. 3, line, i.e. polygon,A4F5F6F4 excluding line F6F4 encompasses and defines the composition ofthe sintered body produced by the present process; and

FIG. 4 is an enlarged view of the section of FIG. 3 showing polygonA4F5F6F4.

FIGS. 1 and 3 show the same composition diagram showing the subsolidusphase equilibria in the reciprocal ternary system comprised of AlN, YN,Y₂ O₃ and Al₂ O₃ and differ in that FIG. 1 shows the polygon ABCDEF ofU.S. Pat. No. 4,547,471 and the line ZZ', whereas FIG. 3 shows thepolygon A4F5F6F4. The composition defined and encompassed by the polygonABCDEF does not include the composition of the present invention.

FIGS. 1 and 2 were developed algebraically on the basis of data producedby forming a particulate mixture of YN of predetermined oxygen contentand AlN powder of predetermined oxygen content, and in a few instances amixture of AlN, YN and Y₂ O₃ powders, under nitrogen gas, shaping themixture into a compact under nitrogen gas and sintering the compact fortime periods ranging from 1 to 1.5 hours at sintering temperaturesranging from about 1860° C. to about 2050° C. in nitrogen gas at ambientpressure. More specifically, the entire procedure ranging from mixing ofthe powders to sintering the compact formed therefrom was carried out ina nonoxidizing atmosphere of nitrogen.

Polygon A4F5F6F4 of FIGS. 3 and 4 also was developed algebraically onthe basis of data produced by the examples set forth herein as well asother experiments which included runs carried out in a manner similar tothat of the present examples.

The best method to plot phase equilibria that involve oxynitrides andtwo different metal atoms, where the metal atoms do not change valence,is to plot the compositions as a reciprocal ternary system as is done inFIGS. 1 and 3. In the particular system of FIGS. 1 and 3 there are twotypes of non-metal atoms (oxygen and nitrogen) and two types of metalatoms (yttrium and aluminum). The Al, Y, oxygen and nitrogen are assumedto have a valence of 3, +3, -2, and -3, respectively. All of the Al, Y,oxygen and nitrogen are assumed to be present as oxides, nitrides oroxynitrides, and to act as if they have the aforementioned valences.

The phase diagrams of FIGS. 1 to 4 are plotted in equivalent percent.The number of equivalents of each of these elements is equal to thenumber of moles of the particular element multiplied by its valence.Along the ordinate is plotted the number of oxygen equivalentsmultiplied by 100% and divided by the sum of the oxygen equivalents andthe nitrogen equivalents. Along the abscissa is plotted the number ofyttrium equivalents multiplied by 100% and divided by the sum of theyttrium equivalents and the aluminum equivalents. All compositions ofFIGS. 1 to 4 are plotted in this manner.

Compositions on the phase diagrams of FIGS. 1 to 4 can also be used todetermine the weight percent and the volume percent of the variousphases. For example, a particular point in the polygon A4F5F6F4 in FIGS.3 or 4 can be used to determine the phase composition of thepolycrystalline body at that point.

FIGS. 1 to 4 show the composition and the phase equilibria of thepolycrystalline body in the solid state.

In U.S. Pat. No. 4,547,471 entitled "High Thermal Conductivity AluminumNitride Ceramic Body" assigned to the assignee hereof and incorporatedherein by reference, there is disclosed the process for producing apolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by line ABCDEF but not including lines CD and EFof FIG. 1 therein (also shown as prior art FIG. 1 herein), a porosity ofless than about 10% by volume of said body and a thermal conductivitygreater than 1.0 W/cm.K at 22° C. which comprises forming a mixturecomprised of aluminum nitride powder and an yttrium additive selectedfrom the group consisting of yttrium, yttrium hydride, yttrium nitrideand mixtures thereof, said aluminum nitride and yttrium additive havinga predetermined oxygen content, said mixture having a compositionwherein the equivalent % of yttrium, aluminum, nitrogen and oxygen isdefined and encompassed by line ABCDEF but not including lines CD and EFin FIG. 1, shaping said mixture into a compact, and sintering saidcompact at a temperature ranging from about 1850° C. to about 2170° C.in an atmosphere selected from the group consisting of nitrogen, argon,hydrogen and mixtures thereof to produce said polycrystalline body.

U.S. Pat. No. 4,547,471 also discloses a polycrystalline body having acomposition comprised of from greater than about 1.6 equivalent %yttrium to about 19.75 equivalent % yttrium, from about 80.25 equivalent% aluminum up to about 98.4 equivalent % aluminum, from greater thanabout 4.0 equivalent % oxygen to about 15.25 equivalent % oxygen andfrom about 84.75 equivalent % nitrogen up to about 96 equivalent %nitrogen.

U.S. Pat. No. 4,547,471 also discloses a polycrystalline body having aphase composition comprised of AlN and a second phase containing Y and Owherein the total amount of said second phase ranges from greater thanabout 4.2% by volume to about 27.3% by volume of the total volume ofsaid body, said body having a porosity of less than about 10% by volumeof said body and a thermal conductivity greater than 1.0 W/cm.K at 22°C.

Briefly stated, the present process for producing the present sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by line, i.e. polygon, A4F5F6F4 excluding lineF6F4 of FIGS. 3 or 4, a porosity of less than about 10% by volume ofsaid body and a thermal conductivity greater than 0.90 W/cm.K at 25° C.comprises the steps:

(a) forming a mixture comprised of aluminum nitride powder containingoxygen, yttrium oxide or precursor therefor, and a carbonaceous additiveselected from the group consisting of free carbon, a carbonaceousorganic material and mixtures thereof, said carbonaceous organicmaterial thermally decomposing at a temperature ranging from about 50°C. to about 1000° C. to free carbon and gaseous product of decompositionwhich vaporizes away, shaping said mixture into a compact, said mixtureand said compact having a composition wherein the equivalent % ofyttrium and aluminum ranges from point F5 up to point F4 of FIGS. 3 or4, which is from about 2.1 equivalent % to greater than about 0.25equivalent % yttrium and from about 97.9 equivalent % to less than about99.75 equivalent % aluminum, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon A4F5F6F4 of FIGS. 3 or 4,

(b) heating sad compact in a nonoxidizing atmosphere at a temperature upto about 1200° C. thereby providing yttrium oxide and free carbon,

(c) heating said compact in a nitrogen-containing nonoxidizingatmosphere at a temperature ranging from about 1350° C. to a temperaturesufficient to deoxidize the compact but below its pore closingtemperature reacting said free carbon with oxygen contained in saidaluminum nitride producing a deoxidized compact, said deoxidized compacthaving a composition wherein the equivalent % of Al, Y, O and N isdefined and encompassed by polygon A4F5F6F4 excluding line F6F4 of FIGS.3 or 4, said free carbon being in an amount which produces saiddeoxidized compact, and

(d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a temperature of at least about 1830° C.producing said polycrystalline body.

In the present process, the composition of the deoxidized compact inequivalent % is the same as or does not differ significantly from thatof the resulting sintered body in equivalent %.

In the present invention, oxygen content can be determined by neutronactivation analysis.

By weight % or % by weight of a component herein, it is meant that thetotal weight % of all the components is 100%.

By ambient pressure herein, it is meant atmospheric or about atmosphericpressure.

By specific surface area or surface area of a powder herein, it is meantthe specific surface area according to BET surface area measurement.

In one embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon A4FF2F4 but excluding line F2F4 ofFIG. 4, the mixture and compact have a composition wherein theequivalent % of yttrium and aluminum ranges from point F up to point F4of FIG. 4, i.e. the yttrium ranges from about 1.6 equivalent % togreater than about 0.25 equivalent % and the aluminum ranges from about98.4 equivalent % to less than about 99.75 equivalent %, and the compactbefore deoxidation has an equivalent % composition of Y, Al, O and Noutside the composition defined and encompassed by polygon A4FF2F4 ofFIG. 4.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon F1FF2F3 but excluding line F2F3 ofFIG. 4, the mixture and compact have a composition wherein theequivalent % of yttrium and aluminum ranges from point F up to point F3of FIG. 4, i.e. the yttrium ranges from about 1.6 equivalent % togreater than about 0.5 equivalent % and the aluminum ranges from about98.4 equivalent % to less than about 99.5 equivalent %, and the compactbefore deoxidation has an equivalent % composition of Y, Al, O and Noutside the composition defined and encompassed by polygon F1FF2F3 ofFIG. 4.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon F7F1F3 but excluding point F3 andline F1F3 of FIG. 4, the mixture and compact have a composition whereinthe equivalent % of yttrium and aluminum ranges from point F1 to lineF7F3 excluding point F3 of FIG. 4, i.e. the yttrium ranges from about1.25 equivalent % to about 0.5 equivalent % and the aluminum ranges fromabout 98.75 equivalent % to about 99.5 equivalent %, and the compactbefore deoxidation has an equivalent % composition of Y, Al, O and Noutside the composition defined and encompassed by polygon F7F1F3 ofFIG. 4.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon A4F7F3F4 but excluding line F3F4 ofFIG. 4, the mixture and compact have a composition wherein theequivalent % of yttrium and aluminum ranges from line F7F3 excludingpoint F3 up to point F4 of FIG. 4, i.e. the yttrium ranges from about0.5 equivalent % to greater than about 0.25 equivalent % and thealuminum ranges from about 99.5 equivalent % to less than about 99.75equivalent %, and the compact before deoxidation has an equivalent %.composition of Y, Al, O and N outside the composition defined andencompassed by polygon A4F7F3F4 of FIG. 4.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line FF5 of FIG. 4, the mixture and compact have acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint F to point F5, i.e. the yttrium ranges from about 1.6 equivalent %to about 2.1 equivalent % and the aluminum ranges from about 98.4equivalent % to about 97.9 equivalent %, and the compact beforedeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined by line FF5.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line F1nF of FIG. 4, the mixture and compact have acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint F1 to point F, i.e. the yttrium ranges from about 1.25 equivalent% to about 1.6 equivalent % and the aluminum ranges from about 98.75equivalent % to about 98.4 equivalent %, and the compact beforedeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined by line F1F.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line F7F1 of FIG. 4, the mixture and compact have acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint F7 to point F1, i.e. the yttrium ranges from about 0.5 equivalent% to about 1.25 equivalent % and the aluminum ranges from about 99.5equivalent % to about 98.75 equivalent %, and the compact beforedeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined by line F7F1 of FIG. 4.

In another embodiment of the present process to product a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line A4F7 of FIG. 4, the mixture and compact have acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint A4 to point F7, i.e. the yttrium ranges from about 0.3 equivalent% to about 0.5 equivalent % and the aluminum ranges from about 99.7equivalent % to about 99.5 equivalent %, and the compact beforedeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined by line A4F7 of FIG. 4.

The calculated compositions of particular points in FIGS. 3 or 4 in thepolygon A4F5F6F4 are shown in Table I as follows:

                  TABLE I                                                         ______________________________________                                        Composition                                                                   (Equivalent                                                                   %)              Vol % and (Wt %) of Phases*                                   Point                                                                              Y        Oxygen   AlN      (YAlO.sub.3                                                                         Y.sub.3 Al.sub.5 O.sub.12               ______________________________________                                        A4   0.3      1.4      99.2(98.8)                                                                             0.8(1.2)                                                                            --                                      F    1.6      4.0      95.8(93.8)                                                                             4.2(6.2)                                                                            --                                      F1   1.25     3.3      96.7(95.1)                                                                             3.3(4.9)                                                                            --                                      F5   2.1      4.95     94.5(92.0)                                                                             5.5(8.0)                                                                            --                                      F7   0.5      1.8      98.7(98.0)                                                                             1.3(2.0)                                                                            --                                      F3   0.5      2.5      98.3(97.6)                                                                             --    1.7(2.4)                                F4   0.25     1.85     99.1(98.8)                                                                             --    0.9(1.2)                                F2   0.75     3.2      97.4(96.4)                                                                             --    2.6(3.6)                                F6   1.0      3.8      96.6(95.3)                                                                             --    3.4(4.7)                                ______________________________________                                         *  Wt % is given in parentheses,                                              Vol % is given without parentheses                                       

The polycrystalline aluminum nitride body produced by the presentprocess has a composition defined and encompassed by polygon, i.e. line,A4F5F6F4 but excluding line F6F4 of FIGS. 3 or 4. The sinteredpolycrystalline body 4 produced by the present process has a compositioncomprised of from about 2.1 equivalent % yttrium to greater than about0.25 equivalent % yttrium, from about 97.9 equivalent % aluminum to lessthan about 99.75 equivalent % aluminum, from about 4.95 equivalent %oxygen to about 1.4 equivalent % oxygen and from about 95.05 equivalent% nitrogen to about 98.6 equivalent % nitrogen.

Also, the polycrystalline body having a composition defined andencompassed by polygon A4F5F6F4 but excluding line F6F4 of FIGS. 3 or 4is comprised of an Al phase and a second phase which ranges in amountfrom about 0.8% by volume for a composition at point A4, to about 5.5%by volume for a composition at point F5 of the total volume of thesintered body. The second phase can be comprised of YAlO₃ or a mixtureof Y and YAlO₃. When the second phase is comprised of YAlO₃, i.e. atline A4F5, it ranges in amount from about 0.8% by volume to about 5.5%by volume of the sintered body. However, when the second phase is amixture of second phases comprised of YAlO₃ and Y₃ Al₅ O₁₂, i.e. whenthe polycrystalline body has a composition defined and encompassed bypolygon A4F5F6F4 excluding lines A4F5 and F6F4, the total amount of suchsecond phase mixture ranges from greater than about 0.8% by volume toless than about 5.5% by volume. Specifically, in such second phasemixture both of these second phases are always present in at least atrace amount, i.e. at least an amount detectable by X-ray diffractionanalysis, and in such mixture, the YAlO₃ phase can range from a traceamount to less than about 5.5% by volume of the sintered body, and the Yphase can range from a trace amount to less than about 3.4% by volume ofthe total volume of the sintered body. More specifically, when a mixtureof Y and YAlO₃ phases is present, the amount of YAlO₃ phase decreasesand the amount of Y phase increases as the composition moves away fromline A4F5 toward line F6F4 in FIG. 4. Line F6F4 in FIG. 4 is comprisedof AlN phase and a second phase comprised of Y₃ Al₅ O₁₂.

As can be seen from Table I, the polycrystalline body at point F5composition would have the largest amount of second phase present whichat point F5 would be YAlO₃.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon, i.e. line, A4FF2F4 but excluding line F2F4 ofFIGS. 3 or 4. The sintered polycrystalline body of polygon A4FF2F4 butexcluding line F2F4 of FIGS. 3 or 4 produced by the present process hasa composition comprised of from greater than about 0.25 equivalent %yttrium to about 1.6 equivalent % yttrium, from less than about 99.75equivalent % aluminum to about 98.4 equivalent % aluminum, from about1.4 equivalent % oxygen to about 4.0 equivalent % oxygen and from about98.6 equivalent % nitrogen to about 96.0 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygonA4FF2F4 but excluding line F2F4 of FIGS. 3 or 4 is comprised of an AlNphase and a second phase which ranges in amount from about 0.8% byvolume to about 4.2% by volume of the total volume of the sintered body.The second phase is comprised of YAlO₃ or a mixture of Y and YAlO₃. Whenthe second phase is comprised of YAlO₃, i.e. at line A4F, it ranges inamount from about 0.8% by volume to about 4.2% by volume of the sinteredbody. However, when the second phase is a mixture of second phasescomprised of YAlO₃ and Y₃ Al₅ O₁₂, i.e. when the polycrystalline bodyhas a composition defined and encompassed by polygon A4FF2F4 excludinglines A4F and F2F4, the total amount of such second phase mixture rangesfrom greater than about 0.8% by volume to less than about 4.2% byvolume. Specifically, in such second phase mixture both of these secondphases are always present in at least a trace amount, i.e. at least anamount detectable by X-ray diffraction analysis, and in such mixture,the YAlO₃ phase can range from a trace amount to less than about 4.2% byvolume of the sintered body, and the Y₃ Al₅ O₁₂ phase can range from atrace amount to less than about 2.6% by volume of the total volume ofthe sintered body.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon, i.e. line F1FF2F3 but excluding line F2F3 ofFIGS. 3 or 4. The sintered polycrystalline body of polygon F1FF2F3 butexcluding line F2F3 of FIGS. 3 or 4 produced by the present process hasa composition comprised of from greater than about 0.5 equivalent %yttrium to about 1.6 equivalent % yttrium, from less than about 99.5equivalent % aluminum to about 98.4 equivalent % aluminum, from greaterthan about 2.5 equivalent % oxygen to about 4.0 equivalent % oxygen andfrom less than about 97.5 equivalent % nitrogen to about 96.0 equivalent% nitrogen.

Also, the polycrystalline body having a composition defined andencompassed by polygon F1FF2F3 but excluding line F2F3 of FIGS. 3 or 4is comprised of an AlN phase and a second phase which ranges in amountfrom greater than about 1.7% by volume to about 4.2% by volume of thetotal volume of the sintered body. The second phase can be comprised ofYAlO₃ or a mixture of Y and YAlO₃. When the second phase is comprised ofYAlO₃, i.e. at line F1F, it ranges in amount from about 3.3% by volumeto about 4.2% by volume of the sintered body. However, when the secondphase is a mixture of second phases comprised of YAlO₃ and Y₃ Al₅ O₁₂,i.e. when the polycrystalline body has a composition defined andencompassed by polygon F1FF2F3 excluding lines F1F and F3F2, the totalamount of such second phase mixture ranges from greater than about 1.7%by volume to less than about 4.2% by volume. Specifically, in suchsecond phase mixture, both of these second phases are always present inat least a trace amount, i.e. at least an amount detectable by X-raydiffraction analysis, and in such mixture, the YAlO₃ phase can rangefrom a trace amount to less than about 4.2% by volume of the sinteredbody, and the Y₃ Al₅ O₁₂ phase can range from a trace amount to lessthan about 2.6% by volume of the total volume of the sintered body.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon, i.e. line F7F1F3 but excluding point F3 and lineF1F3 of FIGS. 3 or 4. The sintered polycrystalline body of polygonF7F1F3 but excluding point F3 and line F1F3 of FIGS. 3 or 4 produced bythe present process has a composition comprised of from about 0.5equivalent % yttrium to about 1.25 equivalent % yttrium, from about 99.5equivalent % aluminum to about 98.75 equivalent % aluminum, from about1.8 equivalent % oxygen to about 3.3 equivalent % oxygen and from about98.2 equivalent % nitrogen to about 96.7 equivalent % nitrogen.

Also, the polycrystalline body having a composition defined andencompassed by polygon F7F1F3 but excluding point F3 and line F1F3 ofFIGS. 3 or 4 is comprised of an AlN phase and a second phase whichranges in amount from about 1.3% by volume for a composition at pointF7, to about 3.3% by volume for a composition at point F1 of the totalvolume of the sintered body. The second phase can be comprised of YAlO₃or a mixture of Y and YAlO₃. When the second phase is comprised ofYAlO₃, i.e. at line F1F7, it ranges in amount from about 1.3% by volumeto about 3.3% by volume of the sintered body. However, when the secondphase is a mixture of second phases comprised of YAlO₃ and Y₃ Al₅ O₁₂,i.e. when the polycrystalline body has a composition defined andencompassed by polygon F7F1F3 excluding point F3 and lines F1F7 andF1F3, the total amount of such second phase mixture ranges from greaterthan about 1.3% by volume to less than about 3.3% by volume.Specifically, in such second phase mixture both of these second phasesare always present in at least a trace amount, i.e. at least an amountdetectable by X-ray diffraction analysis, and in such mixture, the YAlO₃phase can range from a trace amount to less than about 3.3% by volume ofthe sintered body, and the Y₃ Al₅ O₁₂ phase can range from a traceamount to less than about 1.7% by volume of the total volume of thesintered body.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon A4F7F3F4 excluding line F3F4 of FIGS. 3 or 4. Thesintered polycrystalline body of polygon A4F7F3F4 excluding line F3F4 ofFIGS. 3 or 4 produced by the present process has a composition comprisedof from greater than about 0.25 equivalent % yttrium to about 0.5equivalent % yttrium, from less than about 99.75 equivalent % aluminumto about 99.5 equivalent % aluminum, from about 1.4 equivalent % oxygento less than about 2.5 equivalent % oxygen and from greater than about97.5 equivalent % nitrogen to about 98.6 equivalent % nitrogen.

Also, the polycrystalline body having a composition defined andencompassed by polygon A4F7F3F4 excluding line F3F4 of FIGS. 3 or 4 iscomprised of an AlN phase and a second phase which ranges in amount fromabout 0.8% by volume for a composition at point A4, to less than about1.7% by volume for a composition next to point F3 of the total volume ofthe sintered body. The second phase can be comprised of YAlO₃ or amixture of Y₃ Al₅ O₁₂ and YAlO₃. When the second phase is comprised ofYAlO₃, i.e. at line A4F7, it ranges in amount from about 0.8% by volumeto about 1.3% by volume of the sintered body. However, when the secondphase is a mixture of second phases comprised of YAlO₃ and Y₃ Al₅ O₁₂,i.e. when the polycrystalline body has a composition defined andencompassed by polygon A4F7F3F4 excluding lines A4F7 and F3F4, the totalamount of such second phase mixture ranges from greater than about 0.8%by volume to less than about 1.7% by volume. Specifically, in suchsecond phase mixture both of these second phases are always present inat least a trace amount, i.e. at least an amount detectable by X-raydiffraction analysis, and in such mixture, the YAlO₃ phase can rangefrom a trace amount to less than about 1.3% by volume of the sinteredbody, and the Y phase can range from a trace amount to less than about1.7% by volume of the total volume of the sintered body.

In another embodiment, the present process produces a sintered bodydefined by line FF5 of FIG. 4 which has a phase composition comprised ofAlN and YAlO₃ wherein the YAlO₃ phase ranges from about 4.2% by volumeto about 5.5% by volume of the body. Line FF5 of FIG. 4 has acomposition comprised of from about 1.6 equivalent % to about 2.1equivalent % yttrium, from about 98.4 equivalent % to about 97.9equivalent % aluminum, from about 4.0 equivalent % to about 4.95equivalent % oxygen and from about 96.0 equivalent % to about 95.05equivalent % nitrogen.

In another embodiment, the present process produces a sintered bodydefined by line F1F of FIG. 4 which has a phase composition comprised ofAlN and YAlO₃ wherein the YAlO₃ phase ranges from about 3.3% by volumeto about 4.2% by volume of the body. Line F1F of FIG. 4 has acomposition comprised of from about 1.25 equivalent % to about 1.6equivalent % yttrium, from about 98.75 equivalent % to about 98.4equivalent % aluminum, from about 3.3 equivalent % to about 4.0equivalent % oxygen and from about 96.7 equivalent % to about 96.0equivalent % nitrogen.

In another embodiment, the present process produces a sintered bodydefined by line F7F1 of FIG. 4 which has a phase composition comprisedof AlN and YAlO₃ wherein the YAlO₃ phase ranges from about 1.3% byvolume to about 3.3% by volume of the body. Line F7F1 of FIG. 4 has acomposition comprised of from about 0.5 equivalent % to about 1.25equivalent % yttrium, from about 99.5 equivalent % to about 98.75equivalent % aluminum, from about 1.8 equivalent % to about 3.3equivalent % oxygen and from about 98.2 equivalent % to about 96.7equivalent % nitrogen.

In another embodiment, the present process produces a sintered bodydefined by line A4F7 of FIG. 4 which has a phase composition comprisedof AlN and YAlO₃ wherein the YAlO₃ phase ranges from about 0.8% byvolume to about 1.3% by volume of the body. Line A4F7 of FIG. 4 has acomposition comprised of from about 0.3 equivalent % to about 0.5equivalent % yttrium, from about 99.7 equivalent % to about 99.5equivalent % aluminum, from about 1.4 equivalent % to about 1.8equivalent % oxygen and from about 98.6 equivalent % to about 98.2equivalent % nitrogen.

In the present process, the aluminum nitride powder can be of commercialor technical grade. Specifically, it should not contain any impuritieswhich would have a significantly deleterious effect on the desiredproperties of the resulting sintered product. The starting aluminumnitride powder used in the present process contains oxygen generallyranging in amount up to about 4.4% by weight and usually ranging fromgreater than about 1.0% by weight to less than about 4.4% weight, i.e.up to about 4.4% by weight. Typically, commercially available aluminumnitride powder contains from about 1.5 weight % (2.6 equivalent %) toabout 3 weight % (5.2 equivalent %) of oxygen and such powders are mostpreferred on the basis of their substantially lower cost.

The oxygen content of aluminum nitride is determinable by neutronactivation analysis.

Generally, the present starting aluminum nitride powder has a specificsurface area which can range widely, and generally it ranges up to about10 m² /g. Frequently, it has a specific surface area greater than about1.0 m² /g, and more frequently of at least about 3.0 m² /g.

Generally, the present aluminum nitride powder in the present mixture,i.e. after the components have been mixed, usually by milling, has aspecific surface area which can range widely, and generally it ranges toabout 10 m² /g. Frequently, it ranges from greater than about 1.0 m² /gto about 10 m² /g, frequently from about 1.0 m² /g to about 5.0 m² /g,and more frequently from about 3.0 m² /g to about 10 m² /g, andpreferably it ranges from about 3.0 m² /g to about 5 m² /g, according toBET surface area measurement. Specifically, the minimum sinteringtemperature of a given composition of the present invention increaseswith increasing particle size of the aluminum nitride.

Generally, the yttrium oxide (Y₂ O₃) additive in the present mixture hasa specific surface area which can range widely. Generally, it is greaterthan about 0.4 m² /g and generally it ranges from greater than about 0.4m² /g to about 10.0 m² /g, usually from about 0.6 m² /g to about 7.0 m²/g, more usually from greater than about 1.0 m² /g to about 6.0 m² /g.

In the practice of this invention, carbon for deoxidation of aluminumnitride powder is provided in the form of free carbon which can be addedto the mixture as elemental carbon, or in the form of a carbonaceousadditive, for example, an organic compound which can thermally decomposeto provide free carbon.

The present carbonaceous additive is selected from the group consistingof free carbon, a carbonaceous organic material and mixtures thereof.The carbonaceous organic material pyrolyzes, i.e. thermally decomposes,completely at a temperature ranging from about 50° C. to about 1000° C.to free carbon and gaseous product of decomposition which vaporizesaway. In a preferred embodiment, the carbonaceous additive is freecarbon, and preferably, it is graphite.

High molecular weight aromatic compounds or materials are the preferredcarbonaceous organic materials for making the present free carbonaddition since they ordinarily give on pyrolysis the required yield ofparticulate free carbon of submicron size. Examples of such aromaticmaterials are a phenolformaldehyde condensate resin known as Novolakwhich is soluble in acetone or higher alcohols, such as butyl alcohol,as well as many of the related condensation polymers or resins such asthose of resorcinol-formaldehyde, aniline-formaldehyde, andcresolformaldehyde. Another satisfactory group of materials arederivatives of polynuclear aromatic hydrocarbons contained in coal tar,such as dibenzanthracene and chrysene. A preferred group are polymers ofaromatic hydrocarbons such as polyphenylene or polymethylphenylene whichare soluble in aromatic hydrocarbons.

The present free carbon has a specific surface area which can rangewidely and need only be at least sufficient to carry out the presentdeoxidation. Generally, it has a specific surface area greater thanabout 10 m² /g, preferably greater than 20 m² /g, more preferablygreater than about 100 m² /g, and still more preferably greater than 150m² /g, according to BET surface area measurement to insure intimatecontact with the AlN powder for carrying out its deoxidation. Mostpreferably, the present free carbon has as high a surface area aspossible. Also, the finer the particle size of the free carbon, i.e. thehigher its surface area, the smaller are the holes or pores it leavesbehind in the deoxidied compact. Generally, the smaller the pores of agiven deoxidized compact, the lower is the amount of liquid phase whichneed be generated at sintering temperature to produce a sintered bodyhaving a porosity of less than about 1% by volume of the body.

By processing of the aluminum nitride powder into a compact fordeoxidation by free carbon, it is meant herein to include all mixing ofthe aluminum nitride powder to produce the present mixture, all shapingof the resulting mixture to produce the compact, as well as handling andstoring of the compact before it is deoxidized by carbon. In the presentprocess, processing of the aluminum nitride powder into a compact fordeoxidation by free carbon is at least partly carried out in air, andduring such processing of the aluminum nitride powder, it picks upoxygen from air usually in an amount greater than about 0.03% by weightof the aluminum nitride, and any such pick up of oxygen is controllableand reproducible or does not differ significantly if carried out underthe same conditions. If desired, the processing of the aluminum nitridepowder into a compact for deoxidation by free carbon can be carried outin air.

In the present processing of aluminum nitride, the oxygen it picks upcan be in any form, i.e. it initially may be oxygen, or initially it maybe in some other form, such as, for example, water. The total amount ofoxygen picked up by aluminum nitride from air or other media generallyis less than about 3.00% by weight, and generally ranges from greaterthan about 0.03% by weight to less than about 3.00% by weight, andusually it ranges from about 0.10% by weight to about 1.00% by weight,and preferably it ranges from about 0.15% by weight to about 0.70% byweight, of the total weight of the aluminum nitride. Generally, thealuminum nitride in the present mixture and compact prior to deoxidationof the compact have an oxygen content of less than about 4.70% byweight, and generally ranges from greater than about 1.00% by weight,and usually greater than about 1.42% by weight to less than about 4.70%by weight, and more usually it ranges from about 2.00% by weight toabout 4.00% by weight, and frequently it ranges from about 2.20% byweight to about 3.50% by weight, of the total weight of aluminumnitride.

The oxygen content of the starting aluminum nitride powder and that ofthe aluminum nitride in the compact prior to deoxidation can bedetermined by neutron activation analysis.

In a compact, an aluminum nitride containing oxygen in an amount ofabout 4.7% by weight or more is not desirable.

In carrying out the present process, a uniform or at least asignificantly uniform mixture or dispersion of the aluminum nitridepowder, yttrium oxide powder and carbonaceous additive, generally in theform of free carbon powder, is formed and such mixture can be formed bya number of techniques. Preferably, the powders are ball milledpreferably in a liquid medium at ambient pressure and temperature toproduce a uniform or significantly uniform dispersion. The millingmedia, which usually are in the form of cylinders or balls, should haveno significant deleterious effect on the powders, and preferably, theyare comprised of steel or polycrystalline aluminum nitride, preferablymade by sintering a compact of milling media size of AlN powder and Y₂O₃ sintering additive. Generally, the milling media has a diameter of atleast about 1/4 inch and usually ranges from about 1/4 inch to about 1/2inch in diameter. The liquid medium should have no significantlydeleterious effect on the powders and preferably it is non-aqueous.Preferably, the liquid mixing or milling medium can be evaporated awaycompletely at a temperature ranging from about room or ambienttemperature to below 300° C. leaving the present mixture. Preferably,the liquid mixing medium is an organic liquid such as heptane, hexane ortrichloroethane. Also, preferably, the liquid milling medium contains adispersant for the aluminum nitride powder thereby producing a uniformor significantly uniform mixture in a significantly shorter period ofmilling time. Such dispersant should be used in a dispersing amount andit should evaporate or decompose and evaporate away completely or leaveno significant residue, i.e. no residue which has a significant effectin the present process, at an elevated temperature below 1000° C.Generally, the amount of such dispersant ranges from about 0.1% byweight to less than about 3% by weight of the aluminum nitride powder,and generally it is an organic liquid, preferably oleic acid.

In using steel milling media, a residue of steel or iron is left in thedried dispersion or mixture which can range from a detectable amount upto about 3.0% by weight of the mixture. This residue of steel or iron inthe mixture has no significant effect in the present process or on thethermal conductivity of the resulting sintered body.

The liquid dispersion can be dried by a number of conventionaltechniques to remove or evaporate away the liquid and produce thepresent particulate mixture. If desired, drying can be carried out inair. Drying of a milled liquid dispersion in air causes the aluminumnitride to pick up oxygen and, when carried out under the sameconditions, such oxygen pick up is reproducible or does not differsignificantly. Also, if desired, the dispersion can be spray dried.

A solid carbonaceous organic material is preferably admixed in the formof a solution to coat the aluminum nitride particles. The solventpreferably is non-aqueous. The wet mixture can then be treated to removethe solvent producing the present mixture. The solvent can be removed bya number of techniques such as by evaporation or by freeze drying, i.e.subliming off the solvent in vacuum from the frozen dispersion. In thisway, a substantially uniform coating of the organic material on thealuminum nitride powder is obtained which on pyrolysis produces asubstantially uniform distribution of free carbon.

The present mixture is shaped into a compact in air, or includesexposing the aluminum nitride in the mixture to air. Shaping of thepresent mixture into a compact can be carried out by a number oftechniques such as extrusion, injection molding, die pressing, isostaticpressing, slip casting, roll compaction or forming or tape casting toproduce the compact of desired shape. Any lubricants, binders or similarshaping aid materials used to aid shaping of the mixture should have nosignificant deteriorating effect on the compact or the present resultingsintered body. Such shaping-aid materials are preferably of the typewhich evaporate away on heating at relatively low temperatures,preferably below 400° C., leaving no significant residue. Preferably,after removal of the shaping aid materials, the compact has a porosityof less than 60% and more preferably less than 50% to promotedensification during sintering.

If the compact contains carbonaceous organic material as a source offree carbon, it is heated at a temperature ranging from about 50° C. toabout 1000° C. to pyrolyze, i.e. thermally decompose, the organicmaterial completely producing the present free carbon and gaseousproduct of decomposition which vaporizes away. Thermal decomposition ofthe carbonaceous organic material is carried out, preferably in a vacuumor at or about ambient pressure, in a nonoxidizing atmosphere.Preferably, the nonoxidizing atmosphere in which thermal decompositionis carried out is selected from the group consisting of nitrogen,hydrogen, a noble gas such as argon and mixtures thereof, and morepreferably it is nitrogen, or a mixture of at least about 25% by volumenitrogen and a gas selected from the group consisting of hydrogen, anoble gas such as argon and mixtures thereof. In one embodiment, it is amixture of nitrogen and from about 0.5% by volume to about 5% by volumehydrogen.

The actual amount of free carbon introduced by pyrolysis of thecarbonaceous organic material can be determined by pyrolyzing theorganic material alone and determining weight loss. Preferably, thermaldecomposition of the organic material in the present compact is done inthe sintering furnace as the temperature is being raised to deoxidizingtemperature, i.e. the temperature at which the resulting free carbonreacts with the oxygen content of the AlN.

Alternately, in the present process, yttrium oxide can be provided bymeans of an yttrium oxide precursor. The term yttrium oxide precursormeans any organic or inorganic compound which decomposes completely at atemperature below about 1200° C. to form yttrium oxide and by-productgas which vaporizes away leaving no contaminants in the sintered bodywhich would be detrimental to the thermal conductivity. Representativeof the precursors of yttrium oxide useful in the present process isyttrium acetate, yttrium carbonate yttrium oxalate, yttrium nitrate,yttrium sulfate and yttrium hydroxide.

If the compact contains a precursor for yttrium oxide, it is heated to atemperature up to about 1200° C. to thermally decompose the precursorthereby providing yttrium oxide. Such thermal decomposition is carriedout in a non-oxidizing atmosphere, preferably in a vacuum or at or aboutambient pressure, and preferably the atmosphere is selected from thegroup consisting of nitrogen, hydrogen, a noble gas such as argon andmixtures thereof. Preferably, it is nitrogen, or a mixture of at leastabout 25% by volume nitrogen and a gas selected from the groupconsisting of hydrogen, a noble gas such as argon and mixtures thereof.In one embodiment, it is a mixture of nitrogen and from about 0.5% byvolume to about 5% by volume hydrogen.

The present deoxidation of aluminum nitride with carbon, i.e.carbon-deoxidation, comprises heating the compact comprised of aluminumnitride, free carbon and yttrium oxide at deoxidation temperature toreact the free carbon with at least a sufficient amount of the oxygencontained in the aluminum nitride to produce a deoxidized compact havinga composition defined and encompassed by polygon A4F5F6F4 but excludingline F6F4 of FIGS. 3 or 4. This deoxidation with carbon is carried outat a temperature ranging from about 1350° C. to a temperature at whichthe pores of the compact remain open, i.e. a temperature which issufficient to deoxidize the compact but below its pore closingtemperature, generally up to about 1800° C., and preferably, it iscarried out at from about 1600° C. to 1650° C.

The carbondeoxidation is carried out, preferably at ambient pressure, ina gaseous nitrogen-containing nonoxidizing atmosphere which containssufficient nitrogen to facilitate the deoxidation of the aluminumnitride. In accordance with the present invention, nitrogen is arequired component for carrying out the deoxidation of the compact.Preferably, the nitrogen-containing atmosphere is nitrogen, or it is amixture of at least about 25% by volume of nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon, andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixtureranging from about 0.5% to about 5% by volume hydrogen.

The time required to carry out the present carbon-deoxidation of thecompact is determinable empirically and depends largely on the thicknessof the compact as well as the amount of free carbon it contains, i.e.the carbon deoxidation time increases with increasing thickness of thecompact and with increasing amounts of free carbon contained in thecompact. Carbon-deoxidation can be carried out as the compact is beingheated to sintering temperature provided that the heating rate allowsthe deoxidation to be completed while the pores of the compact are openand such heating rate is determinable empirically. Also, to some extent,carbon deoxidation time depends on deoxidation temperature, particlesize and uniformity of the particulate mixture of the compact i.e. thehigher the deoxidation temperature, the smaller the particle size andthe more uniform the mixture, the shorter is deoxidation time.Typically, the carbon-deoxidation time ranges from about 1/4 hour toabout 1.5 hours.

Preferably, the compact is deoxidized in the sintering furnace byholding the compact at deoxidation temperature for the required time andthen raising the temperature to sintering temperature. The deoxidationof the compact must be completed before sintering closes off pores inthe compact preventing gaseous product from vaporizing away and therebypreventing production of the present sintered body.

In the present deoxidation with carbon, the free carbon reacts with theoxygen of the aluminum nitride producing carbon monoxide gas whichvaporizes away. It is believed that the following deoxidation reactionoccurs wherein the oxygen content of the aluminum nitride is given asAl₂ O₃ :

    Al.sub.2 O.sub.3 +3C+N.sub.2 →3CO.sub.(g) +2AlN     (2)

In the deoxidation effected by carbon, gaseous carbon-containing productis produced which vaporizes away thereby removing free carbon.

If the compact before deoxidation is heated at too fast a rate throughthe carbon-deoxidation temperature to sintering temperature, and suchtoo fast rate would depend largely on the composition of the compact andthe amount of carbon it contains, the present carbon-deoxidation doesnot occur, i.e. an insufficient amount of deoxidation occurs, and asignificant amount of carbon is lost by reactions (3) and/or (3A).

    C+AlN→AlCN.sub.(g)                                  (3)

    C+1/2N.sub.2 →CN.sub.(g)                            (3A)

The specific amount of free carbon required to produce the presentdeoxidized compact can be determined by a number of techniques. It canbe determined empirically. Preferably, an initial approximate amount ofcarbon is calculated from Equation (2), that is the stoichiometricamount for carbon set forth in Equation (2), and using such approximateamount, the amount of carbon required in the present process to producethe present sintered body would require one or a few runs to determineif too much or too little carbon had been added. Specifically, this canbe done by determining the porosity of the sintered body and byanalyzing it for carbon and by X-ray diffraction analysis. If thecompact contains too much carbon, the resulting deoxidized compact willbe more difficult to sinter and will not produce the present sinteredbody. If the compact contains too little carbon, X-ray diffractionanalysis of the resulting sintered body will show that its compositionis not that of the present product.

The amount of free carbon used to carry out the present deoxidationshould produce the present deoxidized compact leaving no significantamount of carbon in any form, i.e. no amount of carbon in any form whichwould have a significantly deleterious effect on the sintered body. Morespecifically, no amount of carbon in any form should be left in thedeoxidized compact which would prevent production of the presentsintered body, i.e. any carbon content in the sintered body should below enough so that the sintered body has a thermal conductivity greaterthan 0.90 W/.K at 25° C. Generally, the present sintered body maycontain carbon in some form in a trace amount, i.e. generally less thanabout 0.08% by weight, preferably in an amount of less than about 0.065%by weight, more preferably less than about 0.04% by weight, and mostpreferably less than 0.03% by weight of the total weight of the sinteredbody.

A significant amount of carbon in any form remaining in the sinteredbody significantly reduces its thermal conductivity. An amount of carbonin any form greater than about 0.065% by weight of the sintered body islikely to significantly decrease its thermal conductivity.

The present deoxidized compact is densified, i.e. liquid-phase sintered,at a temperature which is a sintering temperature for the composition ofthe deoxidized compact to produce the present polycrystalline bodyhaving a porosity of less than about 10% by volume, and preferably lessthan about 4% by volume, of the sintered body. For the presentcomposition defined and encompassed by polygon A4F5F6F4 but excludingline F6F4, this sintering temperature generally is at least about 1830°C. and generally ranges from about 1830° C. to about 2050° C. with theminimum sintering temperature increasing generally from about 1830° C.for a composition represented by point F5 to greater than about 1900° C.for compositions represented by line A4F4. Minimum sintering temperatureis dependent most strongly on composition and less strongly on particlesize.

More specifically, in the present invention, for the present deoxidizedcompact having a constant particle size, the minimum sinteringtemperature occurs at compositions represented by point F5 of thepolygon A4F5F6F4 and such temperature increases as the composition movesaway from point F5 toward line A4F4.

More specifically, the minimum sintering temperature is dependentlargely on the composition (i.e., position in the FIG. 4 phase diagram),the green density of the compact, i.e. the porosity of the compact afterremoval of shaping aid materials but before deoxidation, the particlesize of aluminum nitride, and to a much lesser extent the particle sizeof yttrium oxide and carbon. The minimum sintering temperature increasesas the composition moves from point F5 to line A4F4, and as the greendensity of the compact decreases, and as the particle size of aluminumnitride and to a much lesser extent, yttrium oxide and carbon increases.

To carry out the present liquid phase sintering, the present deoxidizedcompact contains sufficient equivalent percent of Y and O to form asufficient amount of liquid phase at sintering temperature to densifythe carbon deoxidized compact to produce the present sintered body. Thepresent minimum densification, i.e. sintering, temperature depends onthe composition of the deoxidized compact, i.e. the amount of liquidphase it generates. Specifically, for a sintering temperature to beoperable in the present invention, it must generate at least sufficientliquid phase in the particular composition of the deoxidized compact tocarry out the present liquid phase sintering to produce the presentproduct. For a given composition, the lower the sintering temperature,the smaller is the amount of liquid phase generated, i.e. densificationbecomes more difficult with decreasing sintering temperature. However, asintering temperature higher than about 2050° C. provides no significantadvantage.

The deoxidized compact is sintered, preferably at or about ambientpressure, in a gaseous nitrogen-containing nonoxidizing atmosphere whichcontains at least sufficient nitrogen to prevent significant weight lossof aluminum nitride. In accordance with the present invention, nitrogenis a necessary component of the sintering atmosphere to prevent anysignificant weight loss of AlN during sintering, and also to optimizethe deoxidation treatment and to remove carbon. Significant weight lossof the aluminum nitride can vary depending on its surface area to volumeratio, i.e. depending on the form of the body, for example, whether itis in the form of a thin or thick tape. As a result, generally,significant weight loss of aluminum nitride ranges from in excess ofabout 5% by weight to in excess of about 10% by weight of the aluminumnitride. Preferably, the nitrogen-containing atmosphere is nitrogen, orit is a mixture at least about 25% by volume nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixturecontaining from about 0.5% by volume to about 5% by volume hydrogen.

Sintering time is determinable empirically. Typically, sintering timeranges from about 40 minutes to about 90 minutes.

In one embodiment, i.e. the composition defined by polygon A4F5F6F4 butnot including lines F6F4 and F5A4 of FIG. 4, where the aluminum nitridein the carbon-deoxidized compact contains oxygen, the yttrium oxidefurther deoxidizes the aluminum nitride by reacting with the oxygentherein to form Y₃ Al₅ O₁₂ and YAlO₃, thus decreasing the amount ofoxygen in the AlN lattice to produce the present sintered body having aphase composition comprised of AlN and a second phase mixture comprisedof YAlO₃ and Y₃ Al₅ O₁₂.

In another embodiment, i.e line A4F5 of FIG. 4, where the aluminumnitride in the carbon-deoxidized compact contains a smaller amount ofoxygen, the yttrium oxide further deoxidizes the aluminum nitride byreacting with the oxygen therein to form YAlO₃ and the resultingsintered body has a phase composition comprised of AlN and YAlO₃.

The present sintered polycrystalline body is a pressureless sinteredceramic body. By pressureless sintering herein it is meant thedensification or consolidation of the deoxidized compact without theapplication of mechanical pressure into a ceramic body having a porosityof less than about 10% by volume.

The polycrystalline body of the present invention is liquid-phasesintered. I.e., it sinters due to the presence of a liquid phase, thatis liquid at the sintering temperature and is rich in yttrium and oxygenand contains some aluminum and nitrogen. In the present polycrystallinebody, the AlN grains have about the same dimensions in all directions,and are not elongated or disk shaped. Generally, the AlN in the presentpolycrystalline body has an average grain size ranging from about 1micron to about 20 microns. An intergranular second phase of YAlO₃ or amixture of YAlO₃ and Y₃ Al₅ O₁₂ is present along some of the AlN grainboundaries. The morphology of the microstructure of the present sinteredbody indicates that this intergranular second phase was a liquid at thesintering temperature. As the composition approaches point F5 in FIG. 4,the amount of liquid phase increases and the AlN grains in the presentsintered body become more rounded and have a smoother surface. As thecomposition moves away from point F5 in FIG. 4 and approaches line A4F4,the amount of liquid phase decreases and the AlN grains in the presentsintered body become less rounded and the corners of the grains becomesharper.

The present sintered body has a porosity of less than about 10% byvolume, and generally less than about 4% by volume of the sintered body.Preferably, the present sintered body has a porosity of less than about2% and most preferably less than about 1% by volume of the sinteredbody. Any pores in the sintered body are fine sized, and generally theyare less than about 1 micron in diameter. Porosity can be determined bystandard metallographic procedures and by standard density measurements.

The present process is a control process for producing a sintered bodyof aluminum nitride having a thermal conductivity greater than 0.90W/cm.K at 25° C., and preferably greater than 1.00 W/cm.K at 25° C.Generally, the thermal conductivity of the present polycrystalline bodyis less than that of a high purity single crystal of aluminum nitridewhich is about 2.8 W/cm.K at 25° C. If the same procedure and conditionsare used throughout the present process, the resulting sintered body hasa thermal conductivity and composition which is reproducible or does notdiffer significantly. Generally, thermal conductivity increases with adecrease in volume % of second phase, a decrease in porosity and for agiven composition with increase in sintering temperature.

In the present process, aluminum nitride picks up oxygen in acontrollable or substantially controllable manner. Specifically, if thesame procedure and conditions are used in the present process, theamount of oxygen picked up by aluminum nitride is reproducible or doesnot differ significantly. Also, in contrast to yttrium, yttrium nitrideand yttrium hydride, yttrium oxide or the present precursor does notpick up oxygen, or does not pick up any significant amount of oxygen,from air or other media in the present process. More specifically, inthe present process, yttrium oxide does not pick up any amount of oxygenin any form from the air or other media which would have any significanteffect on the controllability or reproducibility of the present process.Any oxygen which yttrium oxide might pick up in the present process isso small as to have no effect or no significant effect on the thermalconductivity or composition of the resulting sintered body.

Examples of calculations for equivalent % are as follows:

For a starting AlN powder weighing 89.0 grams measured as having 2.3weight % oxygen, it is assumed that all of the oxygen is bound to AlN asAl₂ O₃, and that the measured 2.3 weight % of oxygen is present as 4.89weight % Al₂ O₃ so that the AlN powder is assumed to be comprised of84.65 grams AlN and 4.35 grams Al₂ O₃.

A mixture is formed comprised of 89.0 grams of the starting AlN powder,2.5 grams of Y₂ O₃ and 0.885 grams free carbon.

During processing, this AlN powder picks up additional oxygen byreactions similar to (4) and now contains 2.6 weight % oxygen.

    2 AlN+3H.sub.2 O→Al.sub.2 O.sub.3 +2NH.sub.3        (4)

The resulting compact now is comprised of the following composition:

89.11 grams AlN powder containing 2.6 weight % oxygen, (84.19 g AlN+4.92g Al₂ O₃, 2.5 grams Y₂ O₃ and 0.885 grams carbon.

During deoxidation of the compact, all the carbon is assumed to reactwith Al₂ O₃ via reaction (5)

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO.sub.(g)      (5)

In the present invention, the carbon will not reduce Y₂ O₃, but insteadreduces Al₂ O₃.

After reaction (5) has gone to completion, the deoxidized compact now iscomprised of the following composition which was calculated on the basisof Reaction (5):

88.62 grams AlN powder containing 1.28 weight % oxygen (86.20 gramsAlN+2.42 grams Al₂ O₃) and 2.5 grams Y₂ O₃

From this weight composition, the composition in equivalent % can becalculated as follows

    ______________________________________                                                Wt (g)        Moles   Equivalents                                     ______________________________________                                        AlN     86.20         2.103   6.309                                           Al.sub.2 O.sub.3                                                                      2.42          0.0237  0.142                                           Y.sub.2 O.sub.3                                                                       2.50          0.0111   0.0664                                         ______________________________________                                        TOTAL EQUIVALENTS = 6.518                                                     V = Valence                                                                    ##STR1##                                                                     MW = molecular weight                                                         Eq = Equivalents                                                              Eq = M × V                                                              Valences:                                                                     Al + 3                                                                        Y + 3                                                                         N - 3                                                                         O - 2                                                                         Eq % Y in deoxidized compact = (6)                                             ##STR2##                                                                      ##STR3##                                                                     Eq % O in deoxidized compact = (7)                                             ##STR4##                                                                      ##STR5##                      (8)                                            This deoxidized compact as well as the sintered body contains about 1.02  

To produce the present sintered body containing 0.9 equivalent % Y and3.0 equivalent % O, i.e. comprised of 0.9 equivalent % Y, 99.1equivalent % Al, 3.0 equivalent % O and 97.0 equivalent % N, using anAlN powder measured as having 2.3 weight % Oxygen (4.89 weight % Al ,the following calculations for weight % from equivalent % can be made

100 grams=weight of AlN powder

x grams=weight of Y₂ O₃ powder

z grams=weight of Carbon powder

Assume that during processing, the AlN powder picks up additional oxygenby reaction similar to (9) and in the compact before deoxidation nowcontains 2.6 weight % oxygen (5.52 weight % Al₂ O₃) and weighs 100.12grams

    2AlN+3H.sub.2 O→Al.sub.2 O.sub.3 +2NH.sub.3         (9)

After processing, the compact can be considered as having the followingcomposition:

    ______________________________________                                        Weight (g)       Moles        Equivalents                                     ______________________________________                                        AlN     94.59        2.308        6.923                                       Al.sub.2 O.sub.3                                                                      5.53         0.0542       0.325                                       Y.sub.2 O.sub.3                                                                       x            4.429 × 10.sup.-3 x                                                                  0.02657x                                    C       z            .0833z                                                   ______________________________________                                    

During deoxidation, 3 moles of carbon reduce 1 mole of Al₂ O₃ and in thepresence of N₂ form 2 moles of AlN by the reaction:

    Al.sub.2 O.sub.3 +3C+N.sub.2 →AlN+3CO               (10)

After deoxidation, all the carbon will have reacted and the compact canbe considered as having the following composition:

    ______________________________________                                               Weight (g)  Moles         Equivalents                                  ______________________________________                                        AlN    94.49 + 2.275z                                                                             2.308 + 0.05551z                                                                           6.923 + 0.1665z                              Al.sub.2 O.sub.3                                                                      5.53 - 2.830z                                                                            0.0542 - 0.02775z                                                                           0.325 - 0.1665z                              Y.sub.2 O.sub.3                                                                      x           4.429 × 10.sup.-3 x                                                                   0.02657x                                     ______________________________________                                        T = Total Equivalents = 7.248 + 0.02657x                                       ##STR6##                     (11)                                            Equivalent Fraction of O = 0.030 =                                                                          (12)                                             ##STR7##                                                                     Solving Equations (11) and (12) for x and z:                                  x = 2.48 grams of Y.sub.2 O.sub.3 powder                                      z = 1.03 grams of free carbon                                             

A body in a form or shape useful as a substrate, i.e. in the form of aflat thin piece of uniform thickness, or having no significantdifference in its thickness, usually referred to as a substrate or tape,may become non-flat, for example, warp, during sintering and theresulting sintered body may require a heat treatment after sintering toflatten it out and make it useful as a substrate. This non-flatness orwarping is likely to occur in the sintering of a body in the form of asubstrate or tape having a thickness of less than about 0.070 inch andcan be eliminated by a flattening treatment, i.e. by heating thesintered body, i.e. substrate or tape, under a sufficient appliedpressure at a temperature in the present sintering temperature range offrom about 1830° C. to about 2050° C. for a period of time determinableempirically, and allowing the sandwiched body to cool to below itssintering temperature, preferably to ambient or room temperature, beforerecovering the resulting flat substrate or tape.

Specifically, in one embodiment of this flattening process, the non-flatsubstrate or tape is sandwiched between two plates and may be separatedfrom such plates by a thin layer of AlN powder depending largely on itscomposition and the applied pressure. The sandwiched body is heated toits sintering temperature. i.e. a temperature which is a sinteringtemperature for the sandwiched sintered body, preferably in the sameatmosphere used for sintering, under an applied pressure at leastsufficient to flatten the body, generally at least about 0.03 psi, for atime period sufficient to flatten the sandwiched body, and then thesandwiched body is allowed to cool to below its sintering temperaturebefore it is recovered.

One embodiment for carrying out this flattening treatment of a sinteredthin body or substrate tape comprises sandwiching the sintered non-flatsubstrate or tape between two plates of a material which has nosignificant deleterious effect thereon such as molybdenum or tungsten,or an alloy containing at least about 80% by weight of tungsten ormolybdenum. The sandwiched substrate or tape can be separated from theplates by a thin layer, preferably a discontinuous coating, preferably adiscontinuous monolayer, of aluminum nitride powder preferably justsufficient to prevent the body from sticking to the surfaces of theplates during the flattening heat treatment. The flattening pressure isdeterminable empirically and depends largely on the particular sinteredbody, the particular flattening temperature and flattening time period.The flattening treatment should have no significant deleterious effecton the sintered body. A decrease in flattening temperature requires anincrease in flattening pressure or flattening time. Generally, at atemperature ranging from about 1830° C. to about 2050° C., the appliedflattening pressure ranges from about 0.03 psi to about, 1.0 psi,preferably from about 0.06 psi to about 0.50 psi. Typically, forexample, heating the sandwiched sintered body at the sinteringtemperature under a pressure of from about 0.03 psi to about 0.5 psi for1 hour in nitrogen produces a flat body useful as a substrate,especially as a supporting substrate for a semiconductor such as asilicon chip.

The present invention makes it possible to fabricate simple, complexand/or hollow shaped polycrystalline aluminum nitride ceramic articlesdirectly. Specifically, the present sintered body can be produced in theform of a useful shaped article without machining or without anysignificant machining, such as a hollow shaped article for use as acontainer, a crucible, a thin walled tube, a long rod, a spherical body,a tape, substrate or carrier. It is useful as a sheath for temperaturesensors. It is especially useful as a substrate for a semiconductor suchas a silicon chip. The dimensions of the present sintered body differfrom those of the unsintered body, by the extent of shrinkage, i.e.densification, which occurs during sintering.

The present ceramic body has a number of uses. In the form of a thinflat piece of uniform thickness, or having no significant difference inits thickness, i.e. in the form of a substrate or tape, it is especiallyuseful as packaging for integrated circuits and as a substrate for anintegrated circuit, particularly as a substrate for a semiconducting Sichip for use in computers.

The invention is further illustrated by the following examples whereinthe procedure was as follows, unless otherwise stated:

The starting aluminum nitride powder contained oxygen in an amount ofless than 4% by weight.

The starting aluminum nitride powder was greater than 99% pure AlNexclusive of oxygen.

In all of the examples of Table II, the starting aluminum nitride powderhad a surface area of 3.4 m² /g (0.54 micron) and based on a series ofdeoxidations carried out with carbon powder, it was determined to havecontained about 2.4 wt % oxygen.

In all of the examples of Table II, the Y₂ O₃ powder, before any mixing,i.e. as received, had a surface area of about 4.7 m² /g.

The carbon used in all of the examples of Table II was graphite and ithad a specific surface area of 200 m² /g (0.017 micron) as listed by thevendor.

Trichloroethane was used to carry out the mixing, i.e. milling, of thepowders in all of the examples of Table II.

In all of the examples of Table II, the milling media was AlN cylinders,about 0.5 inches in diameter and 0.5 inches long. The media wasfabricated by die pressing a mixture of AlN powder and about 3 wt % Y₂O₃ powder, along with a binder, and sintering the compacts at about1900° C. in N₂.

In all the Examples of Table II, the AlN, Y₂ O₃ and carbon powders wereimmersed in trichloroethane containing oleic acid in an amount of about0.7% by weight of the aluminum nitride powder in a plastic jar andvibratory milled in the closed jar at room temperature for 14 to 23hours.

In all of the Examples of Table II, the resulting liquid dispersion ofthe given powder mixture was dried in air at ambient pressure for about1 to 3 hours and during such drying, the mixture picked up oxygen fromthe air.

In all of the Examples of Table II, the dried milled powder mixture wasdie pressed at 5 Kpsi to 20 Kpsi in air at room temperature to produce acompact having a density greater than 50% of its theoretical density.

In those examples of Table II, wherein the sintered body is given asbeing of A size the compacts were in the form of a disk, in thoseexamples wherein the sintered body is given as being of C size, thecompacts were in the form of a bar.

In all of the examples of Table II, the given powder mixture as well asthe compact formed therefrom had a composition wherein the equivalent %of yttrium and aluminum ranged between points F5 and F4 of FIG. 4.

The equivalent % composition of Y, Al, O and N of the compacts of all ofthe Examples of Table II, i.e. before deoxidation, was outside thecomposition defined and encompassed by polygon A4F5F6F4 of FIG. 4.

In all of the examples of Table II, the aluminum nitride in the compactbefore deoxidation contained oxygen in an amount of less than about4.70% by weight of the aluminum nitride.

The composition of the deoxidized compacts of all of the Examples ofTable II, except Examples 4-10 and 12, was defined and encompassed bypolygon A4F5F6F4 of FIG. 4 but did not include line F6F4.

In each example of Table II, one compact was formed from the givenpowder mixture and was given the heat treatment shown in Table II.

In all of the examples of Table II, the same atmosphere was used tocarry out the deoxidation of the compacts as was used to carry out thesintering of the deoxidized compact. The atmosphere to carry out thedeoxidization was fed into the furnace at a rate of 1 SCFH to promoteremoval of the gases produced by deoxidation, and the flow rate duringsintering was less than about 0.1 SCFH.

The atmosphere during all of the heat treatment in all of the examplesin Table II was at ambient pressure which was atmospheric or aboutatmospheric pressure.

The furnace was a molybdenum heat element furnace.

The compacts were heated in the furnace to the given deoxidationtemperature at the rate of about 100° C. per minute and then to thegiven sintering temperature at the rate of about 50° C. per minute.

The sintering atmosphere was at ambient pressure, i.e. atmospheric orabout atmospheric pressure.

At the completion of heat treatment, the samples were furnace-cooled toabout room temperature.

All of the examples of Table II were carried out in substantially thesame manner except as indicated in Table II, and except as indicatedherein.

Based on other work, it was known that the sintered bodies produced inTable II had a carbon content of less than about 0.065% by weight.

Based on other experiments, it was estimated that in every example inTable II, the aluminum nitride in the compact before deoxidation had anoxygen content of about by weight to about 0.70% by weight higher thanthat of the starting aluminum nitride powder.

In Table II, the equivalent % composition of the sintered body wascalculated from the starting powder composition and from the X-raydiffraction phase analysis of the sintered body. The Y, Al, N and oxygenare assumed to have their conventional valences of: +3, +3, -3, -2,respectively. In the sintered bodies, the equivalent percent amount of Yand Al was assumed to be the same as that in the starting powder. Theequivalent percent oxygen and nitrogen are approximate and weredetermined from the equivalent percent yttrium and Al and the X-raydiffraction phase analyses of the sintered bodies.

Weight loss in Table II is the difference between the weight of thecompact after die pressing, excluding the weight of the binder, and theresulting sintered body.

Density of the sintered body was determined by the Archimedes method.

Porosity in % by volume of the sintered body was determined by knowingthe theoretical density of the sintered body on the basis of itscomposition and comparing that to the density measured using thefollowing equation: ##EQU2##

The phase composition of each sintered body, except for Examples 8-9,was determined by X-ray diffraction analysis, and the results are shownin Table II. In Table II, "T" indicates less than about 0.6 volume % and"S" indicates from about 0.6 volume % to about 3.6 volume %. Thediffraction lines for the ##EQU3## phase agree with those on card 18-52in the powder diffraction file, for a cubic ##EQU4## having thespinel-type structure.

The thermal conductivity of the sintered body of Examples 2, 3 and 5 wasmeasured at 25° C. by a steady state heat-flow method using a rod shapedsample -0.4 cm×0.4 cm×2.2 cm sectioned from the sintered body. Thismethod was originally devised by A. Berget in 1888 and is described inan article by G. A. Slack in the "Encyclopedia Dictionary of Physics",Ed. by J. Thewlis, Pergamon, Oxford, 1961. In this technique the sampleis placed inside a high-vacuum chamber, heat is supplied at one end byan electrical heater, and the temperatures are measured with fine-wirethermocouples. The sample is surrounded by a guard cylinder. Theabsolute accuracy is about ±3% and the repeatability is about ±1%. As acomparison, the thermal conductivity of an Al₂ O₃ single crystal wasmeasured with a similar apparatus to be 0.44 W/cm.K at about 22° C.

In Table II, the size of the resulting sintered body is given as A or C.The body of A size was in the form of a disk about 0.17 inch inthickness and about 0.32 inch in diameter. The body of C size was in theshape of a bar measuring about 0.16 inch×0.16 inch×1.7 inches.

In all of the examples of Table II, the compact was placed on amolybdenum plate and then given the heat treatment shown in Table II.

In all of the Examples of Tables II wherein the sintered body was of Csize, the starting compact was separated from the molybdenum plate by athin discontinuous layer of AlN powder.

EXAMPLE 1

0.915 grams of Y₂ O₃ powder and 0.400 grams of graphite powder wereadded to 30.00 grams of aluminum nitride powder and the mixture, alongwith aluminum nitride milling media, was immersed in trichloroethanecontaining oleic acid in an amount of about 0.7% by weight of thealuminum nitride powder in a plastic jar and vibratory milled in theclosed jar at room temperature for about 23 hours. The milling media wasthen removed and a binder solution was added to the milled dispersionand the mixture was then roll mixed for about 2 hours. The bindersolution was comprised of 0.6 grams of an organic binder, i.e. polyvinylbutyral which was in the form of a white powder, 0.22 grams of aplasticizer, i.e. dioctylphthalate, and 0.09 grams of a lubricant, i.e.polyethylene propylene oxide, which were dissolved in trichloroethane.The resulting diversion was dried in air for about 2-3 hours and duringsuch drying, the aluminum nitride picked up oxygen from the air. Duringmilling, the mixture picked up 1.17 grams of AlN due to wear of the AlNmilling media.

A portion of the resulting dried mixture was die pressed producing acompact.

The compact was heated in an atmosphere comprised of nitrogen and 2%hydrogen to 1600° C. where it was held for 1 hour, and then thetemperature was raised to 1850° C. where it was held for 1 hour, andthen the sample was furnace cooled to ambient temperature.

This example is shown as Example 1 in Table II. The sintered bodyproduced in Example 1 had a phase composition comprised of AlN, a smallamount of Y₄ Al₂ O₉ and Y₃ Al₅ O₁₂ and a trace amount of YAlO₃. Also, ithad an equivalent % composition comprised of about 3.4% O, (100%-3.4%)or 96.60% N, 1.06% Y and (100%-1.06%) or 98.94% Al.

Examples 2 to 12 were carried out in substantially the same manner asExample 1 except as indicated herein and except as shown in Table II.

    TABLE II      Properties of Sintered body Heat Treatment  Approx-               Ex.     Sam-ple AlNY.sub.2 O.sub.3C(wt %)Powder Mixture PressingPressureKPSI     Temp - Time (°C.)- (Hr)Deoxidation- Temp - Time(°C.)-     (Hr)Sintering Atmos-phere Equivalent %Oxy-Yt-gentrium WeightLoss(%)     Den-sity(g/cc) matePorosity(vol %)      ##STR8##      ThermalConductivity(W/cm.k@25°      C.) Size                             1 210A 95.15 2.82 1.23 20 1600 - 1     + 1850 - 1 N.sub.2 +2% H.sub.2 3.4 1.06 -- 3.30 <1 S T S -- -- C  2 210B     " " " 10 1600 - 1 + 1870 - 1 N.sub.2 +2% H.sub.2 3.4 1.06 -- 3.30 <1 S T     S -- 1.30 C  3 210C " " " 15 1600 - 1 + 1830 - 1 N.sub.2 +2% H.sub.2 3.5     1.06 -- 3.13 5 -- S S -- 1.22 C  4 209A 95.75 2.86 1.40 20 1600 - 1 +     1850 - 1 N.sub.2 +2% H.sub.2 2.8 1.08 -- -- -- T S -- -- -- C  5 202A     95.99 2.88 1.13 20 1600 - 1 + 1850 - 1 N.sub.2 +2% H.sub.2 4.7 1.09 --     3.32 <1 -- T S S 0.62 C  6 197A 96.03 2.92 1.05 20 1600 - 1 + 1850 - 1     N.sub.2 4.4 1.10 -- 3.30 <1 -- T S S -- A  7 197B " " " 20 1600 - 1 +     1850 - 1 N.sub.2 +2% H.sub.2 4.5 1.10 -- 3.31 <1 -- T S S -- C  8 196A     96.24 2.81 0.95 20 1600 - 1 + 1800 - 1 N.sub.2 +2% H.sub.2 -- 1.06 3.2     2.46 25 -- -- -- -- -- A  9 196B " " " 20 1600 - 1 + 1830 - 1 N.sub.2     +2% H.sub.2 -- 1.06 3.3 3.15 4 -- -- -- -- -- A 10 196C " " " 20 1600 -     1 + 1860 - 1 N.sub.2 +2% H.sub.2 4.0 1.06 3.4 3.25 2 -- -- S -- -- A 11     195A 96.08 2.91 1.01 5 1600 - 1 + 1850 - 1 N.sub.2 +2% H.sub.2 3.9 1.10     -- 3.27 1 -- T S -- -- A 12 195A " " " 20 1600 - 1 + 1850 - 1 N.sub.2     4.6 1.10 -- 3.31 <1 -- T S S -- A

Examples 1, 2, 3 and 11 illustrate the present invention. The sinteredbody produced in Examples 1, 2, 3 and 11 is useful for packaging ofintegrated circuits as well as for use as a substrate or carrier for asemiconductor such as a silicon chip.

Examples 1 and 2 of Table II illustrate the present invention eventhough there was a small amount of Y₄ Al₂ O₉ phase formed in thesintered body. Specifically, sectioning of the sintered body of theseexamples showed that this Y₄ Al₂ O₉ phase was located only in the centerof the body, i.e. the Y₄ Al₂ O₉ was a darker grey than the rest of thebody. The formation of this Y₄ Al₂ O₉ phase was caused by an oxygengradient. The sintered bodies normally contain an oxygen gradient withthe ratio of oxygen to nitrogen increasing from the center of the bodyto the outer surface of the body. The oxygen gradient of the sinteredbodies of Examples 1 and 2 appeared to be considerably larger than inthe other samples, and this resulted in the formation of a small amountof Y₄ Al₂ O₉ phase in the center of the body. However, the overallcomposition, i.e. the equivalent % composition, of the sintered body ofExamples 1 and 2 was located within polygon A4F5F6F4 of FIG. 4.

Based on other experiments, and a comparison of Examples 1 and 2, it wasknown that the sintered body produced in Example 1 had a thermalconductivity of about 1.3 W/cm.K at 25° C.

Too much carbon was present in the powder mixture of Example 4, andtherefore, the oxygen content of the sintered body was too low, i.e. ithad a composition below line FA4 of FIG. 4.

In Examples 5-7, an insufficient amount of carbon was present in thepowder mixture resulting in a sintered body with a composition aboveline F4F6F5 of FIG. 4. Example 5 illustrates the sharp drop in thermalconductivity produced outside the present composition above line F4F6 ofFIG. 4.

In Example 8, an insufficient amount of carbon was present in the powdermixture and the sintering temperature was too low resulting in a highlyporous body.

Based on other work and a comparison of Examples 9 and 10, it was knownthat the sintered body produced in Example 9 would have a compositionabove line F5F6 of FIG. 4. In Example 10, an insufficient amount ofcarbon was present in the powder mixture resulting in a sintered bodywith a composition above line F5F6.

Based on other experiments and a comparison with Example 2, it was knownthat the thermal conductivity of the sintered body produced in Example11 was greater than 0.9 W/cm.K at 25° C.

Also, in Example 11, the mixture was die pressed under a pressure whichwas lower than that of the other examples of Table II indicating thatthe decarburization process is more efficient with a compact of lowergreen density.

In U.S. Pat. Nos. 4,478,785 and 4,533,645, entitled HIGH THERMALCONDUCTIVITY ALUMINUM NITRIDE CERAMIC BODY, assigned to the assigneehereof and incorporated herein by reference, there is disclosed theprocess comprising forming a mixture comprised of aluminum nitridepowder and free carbon wherein the aluminum nitride has a predeterminedoxygen content higher than about 0.8% by weight and wherein the amountof free carbon reacts with such oxygen content to produce a deoxidizedpowder or compact having an oxygen content ranging from greater thanabout 0.35% by weight to about 1.1% by weight and which is at least 20%by weight lower than the predetermined oxygen content, heating themixture or a compact thereof to react the carbon and oxygen producingthe deoxidized aluminum nitride, and sintering a compact of thedeoxidized aluminum nitride producing a ceramic body having a densitygreater than 85% of theoretical and a thermal conductivity greater than0.5 W/cm.K at 22° C.

In copending U.S. patent application Ser. No. 722,639, U.S. Pat. No.4,578,234, HIGH THERMAL CONDUCTIVITY CERAMIC BODY, filed on Apr. 12,1985, in the names of Irvin Charles Huseby and Carl Francis Bobik andassigned to the assignee hereof and incorporated herein by reference,there is disclosed the process for producing an aluminum nitride ceramicbody having a composition defined and encompassed by polygon JKLM butnot including line MJ of FIG. 4 of Ser. No. 722,639 and a thermalconductivity greater than 1.00 W/cm.K at 25° C., which comprises forminga mixture comprised of aluminum nitride powder containing oxygen,yttrium oxide, and free carbon, shaping said mixture into a compact,said mixture and said compact having a composition wherein theequivalent % of yttrium and aluminum ranges from point L to less thanpoint J of FIG. 4 of Ser. No. 722,639, said compact having an equivalent% composition of Y, Al, O and N outside the composition defined andencompassed by polygon JKLM of FIG. 4 of Ser. No. 722,639, heating saidcompact up to a temperature at which its pores remain open reacting saidfree carbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonJKLM but not including line MJ of FIG. 4 of Ser. No. 722,639, andsintering said deoxidized compact at a temperature of at least about1860° C. producing said ceramic body.

In copending U.S. patent application Ser. No. 728,624, U.S. Pat. No.4,578,233, entitled HIGH THERMAL CONDUCTIVITY CERAMIC BODY, filed Apr.29, 1985, in the names of Irvin Charles Huseby and Carl Francis Bobikand assigned to the assignee hereof and incorporated herein byreference, there is disclosed the process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon FJDSR but not including line RF of FIG. 4 of Ser. No. 728,624, aporosity of less than about 10% by volume, and a thermal conductivitygreater than 1.00 W/cm.K at 25° C. which comprises forming a mixturecomprised of aluminum nitride powder containing oxygen, yttrium oxide,and free carbon, shaping said mixture into a compact, said mixture andsaid compact having a composition wherein the equivalent % of yttriumand aluminum ranges from point D up to point F of FIG. 4 of Ser. No.728,624, said compact having an equivalent % composition of Y, Al, O andN outside the composition defined and encompassed by polygon FJDSR ofFIG. 4 of Ser. No. 728,624, heating said compact up to a temperature atwhich its pores remain open reacting said free carbon with oxygencontained in said aluminum nitride producing a deoxidized compact, saiddeoxidized compact having a composition wherein the equivalent % of Al,Y, O and N is defined and encompassed by polygon FJDSR but not includingline RF of FIG. 4 of Ser. No. 728,624, and sintering said deoxidizedcompact at a temperature of at least about 1840° C. producing saidceramic body.

In copending U.S. patent application Ser. No. 728,624, U.S. Pat. No.4,578,365, entitled HIGH THERMAL CONDUCTIVITY CERAMIC BODY filed Apr.29, 1985 in the names of Irvin Charles Huseby and Carl Francis Bobik andassigned to the assignee hereof and incorporated herein by reference,there is disclosed the process for producing an aluminum nitride ceramicbody having a composition defined and encompassed by polygon P1N1KJ butnot including lines KJ and P1J of FIG. 4 of Ser. No. 728,117, and athermal conductivity greater than 1.00 W/cm.K at 25° C. which comprisesforming a mixture comprised of aluminum nitride powder containingoxygen, yttrium oxide, and free carbon, shaping said mixture into acompact, said mixture and said compact having a composition wherein theequivalent % of yttrium and aluminum ranges between points K and P1 ofFIG. 4 of Ser. No. 728,117, said compact having an equivalentcomposition of Y, Al, O and N outside the composition defined andencompassed by polygon P1N1KJ of FIG. 4 of Ser. No. 728,117, heatingsaid compact up to a temperature at which its pores remain open reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon PlNlKJ but not including lines KJ and P1J of FIG.4 of Ser. No. 728,117, and sintering said deoxidized compact at atemperature of at least about 1860° C. producing said ceramic body.

In copending U.S. patent application Ser. No. 728,626, U.S. Pat. No.4,578,364, entitled HIGH THERMAL CONDUCTIVITY CERAMIC BODY, filed Apr.29, 1985, in the names of Irvin Charles Huseby and Carl Francis Bobikand assigned to the assignee here of and incorporated herein byreference, there is disclosed the process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon P1JFA4 but not including lines JF and A4F of FIG. 4 of Ser. No.728,626, a porosity of less than about 10% by volume, and a thermalconductivity greater than 1.00 W/cm.K at 25° C. which comprises forminga mixture comprised of aluminum nitride powder containing oxygen,yttrium oxide, and free carbon, shaping said mixture into a compact,said mixture and said compact having a composition wherein theequivalent % of Yttrium and aluminum ranges between points J and A4 ofFIG. 4 of Ser. No. 728,626, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon P1JFA4 of FIG. 4 of Ser. No. 728,626, heatingsaid compact to a temperature at which its pores remain open reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon P1JFA4 but not including lines JF and A4F of FIG.4 of Ser. No. 728,626, and sintering said deoxidized compact at atemperature of at least about 1850° C. producing said ceramic body.

In copending U.S. patent application Ser. No. 728,133, U.S. Pat. No.4,578,232 entitled HIGH THERMAL CONDUCTIVITY CERAMIC BODY, filed on Apr.29, 1985, in the names of Irvin Charles Huseby and Carl Francis Bobikand assigned to the assignee herein and incorporated herein byreference, there is disclosed a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon LT1DM but not including lines LM and DM of FIG. 4 of Ser. No.728,133, a porosity of less than about 10% by volume, and a thermalconductivity higher than 1.00 W/cm.K at 25° C. which comprises forming amixture comprised of aluminum nitride powder containing oxygen, yttriumoxide, and free carbon, shaping said mixture into a compact, saidmixture and said compact having a composition wherein the equivalent %of yttrium and aluminum ranges from point T1 up to point M of FIG. 4 ofSer. No. 728,133, said compact having an equivalent % composition of Y,Al, O and N outside the composition defined and encompassed by polygonLT1DM of FIG. 4 of Ser. No. 728,133, heating said compact to atemperature at which its pores remain open reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonLT1DM but not including lines LM and DM of FIG. 4 of Ser. No. 728,133,and sintering said deoxidized compact at a temperature of at least about1855° C. producing said ceramic body.

What is claimed is:
 1. A process for producing a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon A4F5F6F4 but excluding line F6F4 ofFIG. 4, a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C. which comprisesthe steps:(a) forming a mixture comprised of oxygen-containing aluminumnitride powder, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges from point F5 upto point F4 of FIG. 4, said yttrium ranging from about 2.1 equivalent %to greater than about 0.25 equivalent %, said aluminum ranging fromabout 97.9 equivalent % to less than about 99.75 equivalent %, saidcompact having an equivalent % composition of, Y, Al, O and N outsidethe composition defined and encompassed by polygon A4F5F6F4 of FIG. 4,(b) heating said compact in a nitrogen-containing nonoxidizingatmosphere at a temperature ranging from about 1350° C. to a temperaturesufficient to deoxidize the compact but below its pore closingtemperature reacting said free carbon with oxygen contained in saidaluminum nitride producing a deoxidized compact, said deoxidized compacthaving a composition wherein the equivalent % of Al, Y, O and N isdefined and encompassed by polygon A4F5F6F4 but excluding line F6F4 ofFIG. 4, said free carbon being in an amount which produces saiddeoxidized compact, and (c) sintering said deoxidized compact in anitrogen-containing nonoxidizing atmosphere at a temperature of at leastabout 1830° C. producing said polycrystalline body.
 2. The processaccording to claim 1 wherein said nitrogen-containing atmosphere in step(b) contains sufficient nitrogen to facilitate deoxidation of thealuminum nitride to produce said sintered body.
 3. The process accordingto claim 1 wherein said nitrogen-containing atmosphere in step (c)contains sufficient nitrogen to prevent significant weight loss of saidaluminum nitride.
 4. The process according to claim 1 wherein saidprocess is carried out at about ambient pressure.
 5. The processaccording to claim 1 wherein said aluminum nitride in step (a) has aspecific surface area ranging up to about 10 m² /g and said free carbonhas a specific surface area greater than about 10 m² /g.
 6. The processaccording to claim 1 wherein said process is carried out at aboutambient pressure in an atmosphere comprised of nitrogen and from about0.5% by volume to about 5% by volume of hydrogen.
 7. A process forproducing a sintered polycrystalline aluminum nitride ceramic bodyhaving a composition defined and encompassed by polygon A4FF2F4 butexcluding line F2F4 of FIG. 4, a porosity of less than about 10% byvolume of said body and a thermal conductivity greater than 1.00 W/cm.Kat 25° C. which comprises the steps:(a) forming a mixture comprised ofan oxygen-containing aluminum nitride powder, yttrium oxide, and freecarbon, shaping said mixture into a compact, said mixture and saidcompact having a composition wherein the equivalent % of yttrium andaluminum ranges from point F up to point F4 of FIG. 4, said yttriumranging from about 1.6 equivalent % to greater than about 0.25equivalent %, said aluminum ranging from about 98.4 equivalent % to lessthan about 99.75 equivalent %, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon A4FF2F4 of FIG. 4, (b) heating said compact in anitrogen-containing nonoxidizing atmosphere at a temperature rangingfrom about 1350° C. to a temperature sufficient to deoxidize the compactbut below its pore closing temperature reacting said free carbon withoxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonA4FF2F4 but excluding line F2F4 of FIG. 4, said free carbon being in anamount which produces said deoxidized compact, and (c) sintering saiddeoxidized compact in a nitrogen-containing nonoxidizing atmosphere at atemperature of at least about 1830° C. producing said polycrystallinebody.
 8. The process according to claim 7 wherein saidnitrogen-containing atmosphere in step (b) contains sufficient nitrogento facilitate deoxidation of the aluminum nitride to produce saidsintered body.
 9. The process according to claim 7 wherein saidnitrogen-containing atmosphere in step (c) contains sufficient nitrogento prevent significant weight loss of said aluminum nitride.
 10. Theprocess according to claim 7 wherein said process is carried out atabout ambient pressure.
 11. The process according to claim 7 whereinsaid aluminum nitride in step (a) has a specific surface area ranging upto about 10 m² /g and said free carbon has a specific surface areagreater than about 10 m² /g.
 12. The process according to claim 7wherein said process is carried out at about ambient pressure in anatmosphere comprised of nitrogen and from about 0.5% by volume to about5% by volume of hydrogen.
 13. The process according to claim 7 whereinsaid mixture and said compact have a composition wherein the equivalent% of yttrium and aluminum ranges from point F up to point F3 of FIG. 4,said yttrium ranging from about 1.6 equivalent % to greater than about0.5 equivalent %, said aluminum ranging from about 98.4 equivalent % toless than about 99.5 equivalent %, wherein said compact before saiddeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined and encompassed by polygon F1FF2F3 of FIG. 4,and wherein said sintered body and said deoxidized compact are comprisedof a composition wherein the equivalent percent of Al, Y, O and N isdefined and encompassed by polygon F1FF2F3 but excluding line F2F3 ofFIG.
 4. 14. The process according to claim 7 wherein said mixture andsaid compact have a composition wherein the equivalent % of yttrium andaluminum ranges from point F1 to line F7F3 excluding point F3 of FIG. 4,said yttrium ranging from about 1.25 equivalent % to about 0.5equivalent %, said aluminum ranging from about 98.75 equivalent % toabout 99.5 equivalent %, wherein said compact before said deoxidationhas an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon F7F1F3 of FIG. 4, andwherein said sintered body and said deoxidized compact are comprised ofa composition wherein the equivalent percent of Al, Y, O and N isdefined and encompassed by polygon F7F1F3 but excluding point F3 andline F1F3 of FIG.
 4. 15. The process according to claim 7 wherein saidmixture and said compact have a composition wherein the equivalent % ofyttrium and aluminum ranges from line F7F3 excluding point F3 up topoint F4 of FIG. 4, said yttrium ranging from about 0.5 equivalent % togreater than about 0.25 equivalent %, said aluminum ranging from about99.5 equivalent % to less than about 99.75 equivalent %, wherein saidcompact before said deoxidation has an equivalent % composition of Y,Al, O and N outside the composition defined and encompassed by polygonA4F7F3F4 of FIG. 4, and wherein said sintered body and said deoxidizedcompact are comprised of a composition wherein the equivalent percent ofAl, Y, O and N is defined and encompassed by polygon A4F7F3F4 butexcluding line F3F4 of FIG.
 4. 16. The process according to claim 7wherein said mixture and said compact have a composition wherein theequivalent % of yttrium and aluminum ranges from point F to point F5 ofFIG. 4, said yttrium ranging from about 1.6 equivalent % to about 2.1equivalent %, said aluminum ranging from about 98.4 equivalent % toabout 97.9 equivalent %, wherein said compact before said deoxidationhas an equivalent % composition of Y, Al, O and N outside thecomposition defined by line FF5, and wherein said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Y, O and N is defined by line FF5 of FIG.
 4. 17. Theprocess according to claim 7 wherein said mixture and said compact havea composition wherein the equivalent % of yttrium and aluminum rangesfrom point F1 to point F of FIG. 4, said yttrium ranging from about 1.25equivalent % to about 1.6 equivalent %, said aluminum ranging from about98.75 equivalent % to about 98.4 equivalent %, wherein said compactbefore said deoxidation has an equivalent % composition of Y, Al, O andN outside the composition defined by line F1F, and wherein said sinteredbody and said deoxidized compact are comprised of a composition whereinthe equivalent percent of Al, Y, O and N is defined by line F1F of FIG.4.
 18. The process according to claim 7 wherein said mixture and saidcompact have a composition wherein the equivalent % of yttrium andaluminum ranges from point F7 to point F1 of FIG. 4, said yttriumranging from about 0.5 equivalent % to about 1.25 equivalent %, saidaluminum ranging from about 99.5 equivalent % to about 98.75 equivalent%, wherein said compact before said deoxidation has an equivalent %composition of Y, Al, O and N outside the composition defined by lineF7F1, and wherein said sintered body and said deoxidized compact arecomprised of a composition wherein the equivalent percent of Al, Y, Oand N is defined by line F7F1 of FIG.
 4. 19. The process according toclaim 7 wherein said mixture and said compact have a composition whereinthe equivalent % of yttrium and aluminum ranges from point A4 to pointF7 of FIG. 4, said yttrium ranging from about 0.3 equivalent % to about0.5 equivalent %, said aluminum ranging from about 99.7 equivalent % toabout 99.5 equivalent %, wherein said compact before said deoxidationhas an equivalent % composition of Y, Al, O and N outside thecomposition defined by line A4F7 and wherein said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Y, O and N is defined by line A4F7 of FIG.
 4. 20. Aprocess for producing a sintered polycrystalline aluminum nitrideceramic body having a composition defined and encompassed by polygonA4F5F6F4 but excluding line F6F4 of FIG. 4, a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C. which comprises the steps:(a) forming a mixturecomprised of an oxygen-containing aluminum nitride powder, yttrium oxideor precursor therefor, and a carbonaceous additive selected from thegroup consisting of free carbon, a carbonaceous organic material andmixtures thereof, said carbonaceous organic material thermallydecomposing at a temperature ranging from about 50° C. to about 1000° C.to free carbon and gaseous product of decomposition which vaporizesaway, shaping said mixture into a compact, said mixture and said compacthaving a composition wherein the equivalent % of yttrium and aluminumranges from point F5 up to point F4 of FIG. 4, said yttrium ranging fromabout 2.1 equivalent % to greater than about 0.25 equivalent %, saidaluminum ranging from about 97.9 equivalent % to less than about 99.75equivalent % aluminum, said compact having an equivalent % compositionof Y, Al, O and N outside the composition defined and encompassed bypolygon A4F5F6F4 of FIG. 4, (b) heating said compact in a nonoxidizingatmosphere at a temperature up to about 1200° C. thereby providingyttrium oxide and free carbon, (c) heating said compact in anitrogen-containing nonoxidizing atmosphere at a temperature rangingfrom about 1350° C. to a temperature sufficient to deoxidize the compactbut below its pore closing temperature reacting said free carbon withoxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonA4F5F6F4 but excluding line F6F4 of FIG. 4, said free carbon being in anamount which produces said deoxidized compact, and (d) sintering saiddeoxidized compact in a nitrogen-containing nonoxidizing atmosphere at atemperature of at least about 1830° C. producing said polycrystallinebody.
 21. The process according to claim 20 wherein saidnitrogen-containing atmosphere in step (c) contains sufficient nitrogento facilitate deoxidation of the aluminum nitride to produce saidsintered body.
 22. The process according to claim 20 wherein saidnitrogen-containing atmosphere in step (d) contains sufficient nitrogento prevent significant weight loss of said aluminum nitride.
 23. Theprocess according to claim 20 wherein said process is carried out atabout ambient pressure.
 24. The process according to claim 20 whereinsaid aluminum nitride in step (a) has a specific surface area ranging upto about 10 m² /g and said free carbon has a specific surface areagreater than about 10 m² /g.
 25. The process according to claim 20wherein said process is carried out at about ambient pressure in anatmosphere comprised of nitrogen and from about 0.5% by volume to about5% by volume of hydrogen.
 26. A process for producing a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon A4FF2F4 but excluding line F2F4 ofFIG. 4, a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C. which comprisesthe steps:(a) forming a mixture comprised of an oxygen-containingaluminum nitride powder, yttrium oxide or precursor therefor, and acarbonaceous additive selected from the group consisting of free carbon,a carbonaceous organic material and mixtures thereof, said carbonaceousorganic material thermally decomposing at a temperature ranging fromabout 50° C. to about 1000° C. to free carbon and gaseous product ofdecomposition which vaporizes away, shaping said mixture into a compact,said mixture and said compact having a composition wherein theequivalent % of yttrium and aluminum ranges from point F up to point F4of FIG. 4, said yttrium ranging from about 1.6 equivalent % to greaterthan about 0.25 equivalent %, said aluminum ranging from about 98.4equivalent % to less than about 99.75 equivalent % aluminum, saidcompact having an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon A4FF2F4 of FIG. 4, (b)heating said compact in a nonoxidizing atmosphere at a temperature up toabout 1200° C. thereby providing yttrium oxide and free carbon, (c)heating said compact in a nitrogen-containing nonoxidizing atmosphere ata temperature ranging from about 1350° C. to a temperature sufficient todeoxidize the compact but below its pore closing temperature reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon A4FF2F4 but excluding line F2F4 of FIG. 4, saidfree carbon being in an amount which produces said deoxidized compact,and (d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a temperature of at least about 1830° C.producing said polycrystalline body.
 27. The process according to claim26 wherein said nitrogen-containing atmosphere in step (c) containssufficient nitrogen to facilitate deoxidation of the aluminum nitride toproduce said sintered body.
 28. The process according to claim 26wherein said nitrogen-containing atmosphere in step (d) containssufficient nitrogen to prevent significant weight loss of said aluminumnitride.
 29. The process according to claim 26 wherein said process iscarried out at ambient pressure.
 30. The process according to claim 26wherein said aluminum nitride in step (a) has a specific surface arearanging up to about 10 m² /g and said free carbon has a specific surfacearea greater than about 10 m² /g.
 31. The process according to claim 26wherein said process is carried out at about ambient pressure in anatmosphere comprised of nitrogen and from about 0.5% by volume to about5% by volume of hydrogen.
 32. The process according to claim 26 whereinsaid mixture and said compact have a composition wherein the equivalent% of yttrium and aluminum ranges from point F up to point F3 of FIG. 4,said yttrium ranging from about 1.6 equivalent % to greater than about0.5 equivalent %, said aluminum ranging from about 98.4 equivalent % toless than about 99.5 equivalent %, wherein said compact before saiddeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined and encompassed by polygon F1FF2F3 of FIG. 4,and wherein said sintered body and said deoxidized compact are comprisedof a composition wherein the equivalent percent of Al, Y, O and N isdefined and encompassed by polygon F1FF2F3 but excluding line F2F3 ofFIG.
 4. 33. The process according to claim 26 wherein said mixture andsaid compact have a composition wherein the equivalent % of yttrium andaluminum ranges from point F1 to line F7F3 excluding point F3 of FIG. 4,said yttrium ranging from about 1.25 equivalent % to about 0.5equivalent %, said aluminum ranging from about 98.75 equivalent % toabout 99.5 equivalent %, wherein said compact before said deoxidationhas an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon F7F1F3 of FIG. 4, andwherein said sintered body and said deoxidized compact are comprised ofa composition wherein the equivalent percent of Al, Y, O and N isdefined and encompassed by polygon F7F1F3 but excluding point F3 andline F1F3 of FIG.
 4. 34. The process according to claim 26 wherein saidmixture and said compact have a composition wherein the equivalent % ofyttrium and aluminum ranges from line F7F3 excluding point F3 up topoint F4 of FIG. 4, said yttrium ranging from about 0.5 equivalent % togreater than about 0.25 equivalent %, said aluminum ranging from about99.5 equivalent % to less than about 99.75 equivalent %, wherein saidcompact before said deoxidation has an equivalent % composition of Y,Al, O and N outside the composition defined and encompassed by polygonA4F7F3F4 of FIG. 4, and wherein said sintered body and said deoxidizedcompact are comprised of a composition wherein the equivalent percent ofAl, Y, O and N is defined and encompassed by polygon A4F7F3F4 butexcluding line F3F4 of FIG.
 4. 35. The process according to claim 26wherein said mixture and said compact have a composition wherein theequivalent % of yttrium and aluminum ranges from point F to point F5 ofFIG. 4, said yttrium ranging from about 1.6 equivalent % to about 2.1equivalent %, said aluminum ranging from about 98.4 equivalent % toabout 97.9 equivalent %, wherein said compact before said deoxidationhas an equivalent % composition of Y, Al, O and N outside thecomposition defined by line FF5, and wherein said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Y, O and N is defined by line FF5 of FIG.
 4. 36. Theprocess according to claim 26 wherein said mixture and said compact havea composition wherein the equivalent % of yttrium and aluminum rangesfrom point F1 to point F of FIG. 4, said yttrium ranging from about 1.25equivalent % to about 1.6 equivalent %, said aluminum ranging from about98.75 equivalent % to about 98.4 equivalent %, wherein said compactbefore said deoxidation has an equivalent % composition of Y, Al, O andN outside the composition defined by line F1F, and wherein said sinteredbody and said deoxidized compact are comprised of a composition whereinthe equivalent percent of Al, Y, O and N is defined by line F1F of FIG.4.
 37. The process according to claim 26 wherein said mixture and saidcompact have a composition wherein the equivalent % of yttrium andaluminum ranges from point F7 to point F1 of FIG. 4, said yttriumranging from about 0.5 equivalent % to about 1.25 equivalent %, saidaluminum ranging from about 99.5 equivalent % to about 98.75 equivalent%, wherein said compact before said deoxidation has an equivalent %composition of Y, Al, O and N outside the composition defined by lineF7F1, and wherein said sintered body and said deoxidized compact arecomprised of a composition wherein the equivalent percent of Al, Y, Oand N is defined by line F7F1 of FIG.
 4. 38. The process according toclaim 26 wherein said mixture and said compact have a compositionwherein the equivalent % of yttrium and aluminum ranges from point A4 topoint F7 of FIG. 4, said yttrium ranging from about 0.3 equivalent % toabout 0.5 equivalent %, said aluminum ranging from about 99.7 equivalent% to about 99.5 equivalent %, wherein said compact before saiddeoxidation has an equivalent % composition of Y, Al, O and N outsidethe composition defined by line A4F7, and wherein said sintered body andsaid deoxidized compact are comprised of a composition wherein theequivalent percent of Al, Y, O, and N is defined by line A4F7 of FIG. 4.39. A polycrystalline aluminum nitride body having a composition definedand encompassed by polygon A4F5F6F4 but excluding line F6F4 of FIG. 4,said body being comprised of from about 2.1 equivalent % yttrium togreater than about 0.25 equivalent % yttrium, from about 97.9 equivalent% aluminum to less than about 99.75 equivalent % aluminum, from about4.95 equivalent % oxygen to about 1.4 equivalent % oxygen and from about95.05 equivalent % nitrogen to about 98.6 equivalent % nitrogen, saidbody having a porosity of less than about 10% by volume of said body anda thermal conductivity greater than 1.00 W/cm.K at 25° C.
 40. Apolycrystalline aluminum nitride body having a composition defined andencompassed by polygon A4FF2F4 but excluding line F2F4 of FIG. 4, saidbody being comprised of from greater than about 0.25 equivalent %yttrium to about 1.6 equivalent % yttrium, from less about 99.75equivalent aluminum to about 98.4 equivalent % aluminum, from about 1 4equivalent % oxygen to about 4.0 equivalent % oxygen and from about 98.6equivalent % nitrogen to about 96.0 equivalent % nitrogen, said bodyhaving a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 41. Apolycrystalline aluminum nitride body having a composition defined andencompassed by polygon A4FF2F4 but excluding line F2F4 of FIG. 4, saidbody being comprised of from greater than about 0.25 equivalent %yttrium to about 1.6 equivalent % yttrium, from less than about 99.75equivalent aluminum to about 98.4 equivalent % aluminum, from about 1.4equivalent % oxygen to about 4.0 equivalent % oxygen and from about 98.6equivalent % nitrogen to about 96.0 equivalent % nitrogen, said bodyhaving a porosity of less than about 2% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 42. Apolycrystalline aluminum nitride body having a composition defined andencompassed by polygon F1FF2F3 but excluding line F2F3 of FIG. 4, saidbody being comprised of from greater than about 0.5 equivalent % yttriumto about 1.6 equivalent % yttrium, from less than about 99.5 equivalent% aluminum to about 98.4 equivalent % aluminum, from greater than about2.5 equivalent % oxygen to about 4.0 equivalent % oxygen and from lessthan about 9.75 equivalent % nitrogen to about 96.0 equivalent %nitrogen, said body having a porosity of less than about 10% by volumeof said body and a thermal conductivity greater than 6.00 W/cm.K at 25°C.
 43. A polycrystalline aluminum nitride body having a compositiondefined and encompassed by polygon F7F1F3 but excluding point F3 andline F1F3 of FIG. 4, said body being comprised of from about 0.5equivalent % yttrium to about 1.25 equivalent % yttrium, from about 99.5equivalent % aluminum to about 98.75 equivalent % aluminum, from about1.8 equivalent oxygen to about 3.3 equivalent % oxygen and from about98.2 equivalent % nitrogen to about 96.7 equivalent % nitrogen, saidbody having a porosity of less than 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 44. Apolycrystalline aluminum nitride body having a composition defined andencompassed by polygon A4F7F3F4 but excluding line F3F4 of FIG. 4, saidbody being comprised of from greater than about 0.25 equivalent %yttrium to about 0.5 equivalent % yttrium, from less than about 99.75equivalent % aluminum to about 99.5 equivalent % aluminum, from about1.4 equivalent % oxygen to less than about 2.5 equivalent % oxygen andfrom greater than about 97.5 equivalent % nitrogen to about 98.6equivalent % nitrogen, said body having a porosity of less than about10% by volume of said body and a thermal conductivity greater than 1.00W/cm.K at 25° C.
 45. A polycrystalline aluminum nitride body having acomposition defined by line FF5 of FIG. 4, said body being comprised offrom about 1.6 equivalent % to about 2.1 equivalent % yttrium, fromabout 98.4 equivalent % to about 97.9 equivalent % aluminum, from about4.0 equivalent % to about 4.95 equivalent % oxygen and from about 96.0equivalent % to about 95.05 equivalent % nitrogen, said body having aporosity of less than about 10% by volume of said body and a thermalconductivity greater than 1.00 W/cm.K at 25° C.
 46. A polycrystallinealuminum nitride body having a composition defined by line F1F of FIG.4, said body being comprised of from about 1.25 equivalent % to about1.6 equivalent % yttrium, from about 98.75 equivalent % to about 98.4equivalent % aluminum, from about 3.3 equivalent % to about 4.0equivalent % oxygen and from about 96.7 equivalent % to about 96.0equivalent % nitrogen, said body having a porosity of less than about10% by volume of said body and a thermal conductivity greater than 1.00W/cm.K at 25° C.
 47. A polycrystalline aluminum nitride body having acomposition defined by line F7F1 of FIG. 4, said body being comprised offrom about 0.5 equivalent % to about 1.25 equivalent % yttrium, fromabout 99.5 equivalent % to about 98.75 equivalent % aluminum, from about1.8 equivalent % to about 3.3 equivalent % oxygen and from about 98.2equivalent % to about 96.7 equivalent % nitrogen, said body having aporosity of less than about 10% by volume of said body and a thermalconductivity greater than 1.00 W/cm.K at least 25° C.
 48. Apolycrystalline aluminum nitride body having a composition defined byline A4F7 of FIG. 4, said body being comprised of from about 0.3equivalent % to about 0.5 equivalent % yttrium, from about 99.7equivalent % to about 99.5 equivalent % aluminum, from about 1.4equivalent % to about 1.8 equivalent % oxygen and from about 98.6equivalent % to about 98.2 equivalent % nitrogen, said body having aporosity of less than about 10% by volume of said body and a thermalconductivity greater than 1.00 W/cm.K at 25° C.
 49. A polycrystallinealuminum nitride body having a phase composition comprised of AlN, YAlO₃and Y₃ Al₅ O₁₂ wherein the total amount of said YAlO₃ and Y₃ Al₅ O₁₂phases ranges from greater than about 0.8% by volume to less than about5.5% by volume of the total volume of said body, said YAlO₃ phaseranging from a trace amount to less than about 5.5% by volume of thesintered body, said Y₃ Al₅ O₁₂ phase ranging from a trace amount to lessthan about 3.4% by volume of the sintered body, said body having aporosity of less than about 10% by volume of said body and a thermalconductivity greater than 1.00 W/cm.K at 25° C.
 50. A polycrystallinebody having a phase composition comprised of AlN, YAlO₃ and Y₃ Al₅ O₁₂wherein the total amount of said YAlO₃ and Y₃ Al₅ O₁₂ phases ranges fromgreater than about 0.8% by volume to less than about 4.2% by volume ofthe total volume of said body, said YAlO₃ phase ranging from a traceamount to less than about 4.2% by volume of the sintered body, said Y₃Al₅ O₁₂ phase ranging from a trace amount to less than about 2.6% byvolume of the sintered body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C.
 51. A polycrystalline body having a phasecomposition comprised of AlN, YAlO₃ and Y₃ Al₅ O₁₂ wherein the totalamount of said YAlO₃ and Y₃ Al₅ O₁₂ phases ranges from greater thanabout 1.7% by volume to less than about 4.2% by volume of the totalvolume of said body, said YAlO₃ phase ranging from a trace amount toless than about 4.2% by volume of the sintered body, said Y₃ Al₅ O₁₂phase ranging from a trace amount to less than about 2.6% by volume ofthe sintered body, said body having a porosity of less than about 10% byvolume of said body and a thermal conductivity greater than 1.00 W/cm.Kat 25° C.
 52. A polycrystalline body having a phase compositioncomprised of AlN, YAlO₃ and Y₃ Al₅ O₁₂ wherein the total amount of saidYAlO₃ and Y₃ Al₅ O₁₂ phases ranges from greater than about 1.3% byvolume to less than about 3.3% by volume of the total volume of saidbody, said YAlO₃ phase ranging from a trace amount to less than about3.3% by volume of the sintered body, said Y₃ Al₅ O₁₂ phase ranging froma trace amount to less than about 1.7% by volume of the sintered body,said body having a porosity of less than about 10% by volume of saidbody and a thermal conductivity greater than 1.00 W/cm.K at 25° C.
 53. Apolycrystalline body having a phase composition comprised of AlN, YAlO₃and wherein the total amount of said YAlO₃ and Y₃ Al₅ O₁₂ phases rangesfrom greater than about 0.8% by volume to less than about 1.7% by volumeof the total volume of said body, said YAlO₃ phase ranging from a traceamount to less than about 1.3% by volume of the sintered body, said Y₃Al₅ O₁₂ phase ranging from a trace amount to less than about 1.7% byvolume of the sintered body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C.
 54. A polycrystalline body having a phasecomposition comprised of AlN and YAlO₃ wherein the amount of said YAlO₃phase ranges from about 4.2% by volume to about 5.5% by volume of thetotal volume of said body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C.
 55. A polycrystalline body having a phasecomposition comprised of AlN and YAlO₃ wherein the amount of said YAlO₃phase range from about 3.3% by volume to about 4.2% by volume of thetotal volume of said body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than0.90 W/cm.K at 25° C.
 56. A polycrystalline body having a phasecomposition comprised of AlN and YAlO₃ wherein the amount of said YAlO₃phase ranges from about 1.3% by volume to about 3.3% by volume of thetotal volume of said body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C.
 57. A polycrystalline body having a phasecomposition comprised of AlN and YAlO₃ wherein the amount of said YAlO₃phase ranges from about 0.8% by volume to about 1.3% by volume of thetotal volume of said body, said body having a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than1.00 W/cm.K at 25° C.